DOC: Fix incorrect Sphinx cross references in older, whatsnew restructred text files.

[mroeschke]

The top level doc/ directory contains reStructuredText files who's names will end in .rst and are used to build the pandas documentation using Sphinx. These files can contain Sphinx cross-referencing syntax to create a link to where the reference is documented. Specifically, the Python cross-referencing syntax is used. Cross references should reference currently documented pandas objects or objects in other libaries specified in the intersphinx_mapping variable in doc/source/conf.py

When sphinx-build command is run with nitpicky = True set in the doc/source/conf.py, the logs will specify the file location, line number, and invalid cross references. Below is a subset of invalid cross references from older whatsnew files

doc/source/whatsnew/v0.13.0.rst:749: WARNING: py:meth reference target not found: pandas.io.json.json_normalize [ref.meth]
doc/source/whatsnew/v0.13.1.rst:32: WARNING: py:meth reference target not found: pandas.Panel.apply [ref.meth]
doc/source/whatsnew/v0.13.1.rst:241: WARNING: py:meth reference target not found: pandas.Panel.apply [ref.meth]
doc/source/whatsnew/v0.13.1.rst:339: WARNING: py:meth reference target not found: pandas.Panel.apply [ref.meth]
doc/source/whatsnew/v0.14.0.rst:330: WARNING: py:class reference target not found: pandas.io.parsers.ParserWarning [ref.class]
doc/source/whatsnew/v0.14.0.rst:455: WARNING: py:func reference target not found: create_engine [ref.func]
doc/source/whatsnew/v0.14.0.rst:643: WARNING: py:func reference target not found: parallel_coordinates [ref.func]
doc/source/whatsnew/v0.14.0.rst:647: WARNING: py:func reference target not found: parallel_coordinates [ref.func]
doc/source/whatsnew/v0.14.0.rst:647: WARNING: py:func reference target not found: andrews_curves [ref.func]
doc/source/whatsnew/v0.14.0.rst:664: WARNING: py:class reference target not found: DateRange [ref.class]
doc/source/whatsnew/v0.14.0.rst:672: WARNING: py:meth reference target not found: HDFStore.select_column [ref.meth]
doc/source/whatsnew/v0.14.0.rst:673: WARNING: py:func reference target not found: Timestamp.offset [ref.func]
doc/source/whatsnew/v0.14.0.rst:674: WARNING: py:func reference target not found: get_data_yahoo [ref.func]
doc/source/whatsnew/v0.14.0.rst:674: WARNING: py:func reference target not found: get_data_google [ref.func]
doc/source/whatsnew/v0.14.0.rst:678: WARNING: py:func reference target not found: rolling_sum [ref.func]
doc/source/whatsnew/v0.14.0.rst:732: WARNING: py:meth reference target not found: Panel.shift [ref.meth]
doc/source/whatsnew/v0.14.0.rst:738: WARNING: py:meth reference target not found: Panel.major_xs [ref.meth]
doc/source/whatsnew/v0.14.0.rst:738: WARNING: py:meth reference target not found: Panel.minor_xs [ref.meth]
doc/source/whatsnew/v0.14.0.rst:742: WARNING: py:func reference target not found: parallel_coordinates [ref.func]
doc/source/whatsnew/v0.14.0.rst:746: WARNING: py:func reference target not found: parallel_coordinates [ref.func]
doc/source/whatsnew/v0.14.0.rst:746: WARNING: py:func reference target not found: andrews_curves [ref.func]
doc/source/whatsnew/v0.14.0.rst:759: WARNING: py:func reference target not found: boxplot [ref.func]
doc/source/whatsnew/v0.14.0.rst:848: WARNING: py:func reference target not found: rolling_max [ref.func]
doc/source/whatsnew/v0.14.0.rst:848: WARNING: py:func reference target not found: rolling_min [ref.func]
doc/source/whatsnew/v0.15.0.rst:400: WARNING: py:func reference target not found: rolling_min [ref.func]
doc/source/whatsnew/v0.15.0.rst:400: WARNING: py:func reference target not found: rolling_max [ref.func]
doc/source/whatsnew/v0.15.0.rst:400: WARNING: py:func reference target not found: rolling_cov [ref.func]
doc/source/whatsnew/v0.15.0.rst:400: WARNING: py:func reference target not found: rolling_corr [ref.func]
doc/source/whatsnew/v0.15.0.rst:427: WARNING: py:func reference target not found: rolling_max [ref.func]
doc/source/whatsnew/v0.15.0.rst:427: WARNING: py:func reference target not found: rolling_min [ref.func]
doc/source/whatsnew/v0.15.0.rst:427: WARNING: py:func reference target not found: rolling_sum [ref.func]
doc/source/whatsnew/v0.15.0.rst:427: WARNING: py:func reference target not found: rolling_mean [ref.func]
doc/source/whatsnew/v0.15.0.rst:427: WARNING: py:func reference target not found: rolling_median [ref.func]
doc/source/whatsnew/v0.15.0.rst:427: WARNING: py:func reference target not found: rolling_std [ref.func]
doc/source/whatsnew/v0.15.0.rst:427: WARNING: py:func reference target not found: rolling_var [ref.func]
doc/source/whatsnew/v0.15.0.rst:427: WARNING: py:func reference target not found: rolling_skew [ref.func]
doc/source/whatsnew/v0.15.0.rst:427: WARNING: py:func reference target not found: rolling_kurt [ref.func]
doc/source/whatsnew/v0.15.0.rst:427: WARNING: py:func reference target not found: rolling_quantile [ref.func]
doc/source/whatsnew/v0.15.0.rst:427: WARNING: py:func reference target not found: rolling_cov [ref.func]
doc/source/whatsnew/v0.15.0.rst:427: WARNING: py:func reference target not found: rolling_corr [ref.func]
doc/source/whatsnew/v0.15.0.rst:427: WARNING: py:func reference target not found: rolling_corr_pairwise [ref.func]
doc/source/whatsnew/v0.15.0.rst:427: WARNING: py:func reference target not found: rolling_window [ref.func]
doc/source/whatsnew/v0.15.0.rst:427: WARNING: py:func reference target not found: rolling_apply [ref.func]
doc/source/whatsnew/v0.15.0.rst:433: WARNING: py:func reference target not found: rolling_apply [ref.func]
doc/source/whatsnew/v0.15.0.rst:462: WARNING: py:func reference target not found: rolling_window [ref.func]
doc/source/whatsnew/v0.15.0.rst:496: WARNING: py:func reference target not found: expanding_apply [ref.func]
doc/source/whatsnew/v0.15.0.rst:499: WARNING: py:func reference target not found: expanding_cov [ref.func]
doc/source/whatsnew/v0.15.0.rst:499: WARNING: py:func reference target not found: rolling_cov [ref.func]
doc/source/whatsnew/v0.15.0.rst:502: WARNING: py:func reference target not found: expanding_var [ref.func]
doc/source/whatsnew/v0.15.0.rst:502: WARNING: py:func reference target not found: expanding_std [ref.func]
doc/source/whatsnew/v0.15.0.rst:502: WARNING: py:func reference target not found: rolling_var [ref.func]
doc/source/whatsnew/v0.15.0.rst:502: WARNING: py:func reference target not found: rolling_std [ref.func]
doc/source/whatsnew/v0.15.0.rst:506: WARNING: py:func reference target not found: ewma [ref.func]
doc/source/whatsnew/v0.15.0.rst:506: WARNING: py:func reference target not found: ewmstd [ref.func]
doc/source/whatsnew/v0.15.0.rst:506: WARNING: py:func reference target not found: ewmvol [ref.func]
doc/source/whatsnew/v0.15.0.rst:506: WARNING: py:func reference target not found: ewmvar [ref.func]
doc/source/whatsnew/v0.15.0.rst:506: WARNING: py:func reference target not found: ewmcov [ref.func]
doc/source/whatsnew/v0.15.0.rst:506: WARNING: py:func reference target not found: ewmcorr [ref.func]
doc/source/whatsnew/v0.15.0.rst:506: WARNING: py:func reference target not found: rolling_* [ref.func]
doc/source/whatsnew/v0.15.0.rst:506: WARNING: py:func reference target not found: expanding_* [ref.func]
doc/source/whatsnew/v0.15.0.rst:545: WARNING: py:func reference target not found: ewmstd [ref.func]
doc/source/whatsnew/v0.15.0.rst:545: WARNING: py:func reference target not found: ewmvol [ref.func]
doc/source/whatsnew/v0.15.0.rst:545: WARNING: py:func reference target not found: ewmvar [ref.func]
doc/source/whatsnew/v0.15.0.rst:545: WARNING: py:func reference target not found: ewmcov [ref.func]
doc/source/whatsnew/v0.15.0.rst:545: WARNING: py:func reference target not found: ewmcorr [ref.func]
doc/source/whatsnew/v0.15.0.rst:545: WARNING: py:func reference target not found: ewma [ref.func]
doc/source/whatsnew/v0.15.0.rst:551: WARNING: py:func reference target not found: ewma [ref.func]
doc/source/whatsnew/v0.15.0.rst:551: WARNING: py:func reference target not found: ewmstd [ref.func]
doc/source/whatsnew/v0.15.0.rst:551: WARNING: py:func reference target not found: ewmvol [ref.func]
doc/source/whatsnew/v0.15.0.rst:551: WARNING: py:func reference target not found: ewmvar [ref.func]
doc/source/whatsnew/v0.15.0.rst:551: WARNING: py:func reference target not found: ewmcov [ref.func]
doc/source/whatsnew/v0.15.0.rst:551: WARNING: py:func reference target not found: ewmcorr [ref.func]
doc/source/whatsnew/v0.15.0.rst:584: WARNING: py:func reference target not found: ewm* [ref.func]
doc/source/whatsnew/v0.15.0.rst:589: WARNING: py:func reference target not found: expanding_cov [ref.func]
doc/source/whatsnew/v0.15.0.rst:589: WARNING: py:func reference target not found: expanding_corr [ref.func]
doc/source/whatsnew/v0.15.0.rst:589: WARNING: py:func reference target not found: rolling_cov [ref.func]
doc/source/whatsnew/v0.15.0.rst:589: WARNING: py:func reference target not found: rolling_cor [ref.func]
doc/source/whatsnew/v0.15.0.rst:589: WARNING: py:func reference target not found: ewmcov [ref.func]
doc/source/whatsnew/v0.15.0.rst:589: WARNING: py:func reference target not found: ewmcorr [ref.func]
doc/source/whatsnew/v0.15.0.rst:593: WARNING: py:func reference target not found: rolling_count [ref.func]
doc/source/whatsnew/v0.15.0.rst:593: WARNING: py:func reference target not found: expanding_* [ref.func]
doc/source/whatsnew/v0.15.0.rst:594: WARNING: py:func reference target not found: rolling_apply [ref.func]
doc/source/whatsnew/v0.15.0.rst:594: WARNING: py:func reference target not found: expanding_apply [ref.func]
doc/source/whatsnew/v0.15.0.rst:595: WARNING: py:func reference target not found: expanding_std [ref.func]
doc/source/whatsnew/v0.15.0.rst:595: WARNING: py:func reference target not found: expanding_var [ref.func]
doc/source/whatsnew/v0.15.0.rst:596: WARNING: py:func reference target not found: rolling_std [ref.func]
doc/source/whatsnew/v0.15.0.rst:596: WARNING: py:func reference target not found: rolling_var [ref.func]
doc/source/whatsnew/v0.15.0.rst:598: WARNING: py:func reference target not found: ewmstd [ref.func]
doc/source/whatsnew/v0.15.0.rst:598: WARNING: py:func reference target not found: ewmvol [ref.func]
doc/source/whatsnew/v0.15.0.rst:598: WARNING: py:func reference target not found: ewmvar [ref.func]
doc/source/whatsnew/v0.15.0.rst:598: WARNING: py:func reference target not found: ewmcov [ref.func]
doc/source/whatsnew/v0.15.0.rst:1039: WARNING: py:func reference target not found: describe [ref.func]
doc/source/whatsnew/v0.15.0.rst:1160: WARNING: py:func reference target not found: set_names [ref.func]
doc/source/whatsnew/v0.15.0.rst:1160: WARNING: py:func reference target not found: set_labels [ref.func]
doc/source/whatsnew/v0.15.0.rst:1160: WARNING: py:func reference target not found: set_levels [ref.func]
doc/source/whatsnew/v0.15.0.rst:1160: WARNING: py:func reference target not found: set_names [ref.func]
doc/source/whatsnew/v0.15.1.rst:280: WARNING: py:func reference target not found: io.wb.download [ref.func]
doc/source/whatsnew/v0.16.0.rst:106: WARNING: py:meth reference target not found: SparseSeries.to_coo [ref.meth]
doc/source/whatsnew/v0.16.0.rst:106: WARNING: py:meth reference target not found: SparseSeries.from_coo [ref.meth]
doc/source/whatsnew/v0.17.0.rst:267: WARNING: py:meth reference target not found: pandas.io.read_sas [ref.meth]
doc/source/whatsnew/v0.17.0.rst:356: WARNING: py:func reference target not found: pandas.io.gbq.to_gbq [ref.func]
doc/source/whatsnew/v0.17.0.rst:357: WARNING: py:func reference target not found: pandas.io.gbq.to_gbq [ref.func]
doc/source/whatsnew/v0.17.1.rst:85: WARNING: py:class reference target not found: py:py._path.local.LocalPath [ref.class]
doc/source/whatsnew/v0.17.1.rst:126: WARNING: py:func reference target not found: pandas.io.gbq.to_gbq [ref.func]
doc/source/whatsnew/v0.18.0.rst:834: WARNING: py:meth reference target not found: Resampler.backfill [ref.meth]
doc/source/whatsnew/v0.18.0.rst:834: WARNING: py:meth reference target not found: Resampler.ffill [ref.meth]
doc/source/whatsnew/v0.18.0.rst:834: WARNING: py:meth reference target not found: Resampler.fillna [ref.meth]
doc/source/whatsnew/v0.18.0.rst:834: WARNING: py:meth reference target not found: Resampler.asfreq [ref.meth]
doc/source/whatsnew/v0.19.0.rst:31: WARNING: py:func reference target not found: union_categorical [ref.func]
doc/source/whatsnew/v0.19.0.rst:301: WARNING: py:func reference target not found: union_categoricals [ref.func]
doc/source/whatsnew/v0.19.0.rst:418: WARNING: py:func reference target not found: read_gbq [ref.func]
doc/source/whatsnew/v0.19.0.rst:419: WARNING: py:func reference target not found: DataFrame.to_gbq [ref.func]
doc/source/whatsnew/v0.19.0.rst:499: WARNING: py:func reference target not found: datetime.datetime [ref.func]
doc/source/whatsnew/v0.19.0.rst:1431: WARNING: py:meth reference target not found: Panel4D.to_xarray [ref.meth]

Fix only this subset of references so that sphinx-build will no longer raise a warning. If the object being referenced cannot be corrected to link to a currently documented pandas or intersphinx_mapping library object, replace the cross referncing syntax with double backticks, for example :func:`does_not_exist` should be replaced with ``does_not_exist``.