docs: update bigframes.pandas.Series docs (#1273)

* docs: update bigframes.pandas.Series docs

* update the rest of the methods

* fix formatting

* fix indentation error

* fix some docstrings and a doctest error

* fix formatting

* fix indentation and blankline error

* fix indentation

* fix indentation

* fix kokoro doctest errors

* fix doctest

* fix kokoro doctest

* fix kokoro doctest

* remove .to_string code example

Author: arwas11

third_party/bigframes_vendored/pandas/core/frame.py

@@ -6799,9 +6799,9 @@ def iat(self):
 
             >>> import bigframes.pandas as bpd
             >>> bpd.options.display.progress_bar = None
+
             >>> df = bpd.DataFrame([[0, 2, 3], [0, 4, 1], [10, 20, 30]],
             ...                    columns=['A', 'B', 'C'])
-            >>> bpd.options.display.progress_bar = None
             >>> df
                 A       B       C
             0   0       2       3
@@ -6833,9 +6833,9 @@ def at(self):
 
             >>> import bigframes.pandas as bpd
             >>> bpd.options.display.progress_bar = None
+
             >>> df = bpd.DataFrame([[0, 2, 3], [0, 4, 1], [10, 20, 30]],
             ...   index=[4, 5, 6], columns=['A', 'B', 'C'])
-            >>> bpd.options.display.progress_bar = None
             >>> df
                 A   B   C
             4   0   2   3