Removed skipna argument from count, any, all [GH755]

[sjvrijn]

• Closes count docstring mistakenly includes skipna #755
• User visible changes (including notable bug fixes) are documented in whats-new.rst

Could not confirm in the web-based documentation (see #4257), but manually confirmed to work:

>>> import xarray as xr
>>> help(xr.DataArray.count)
Help on function count in module xarray.core.common:

count(self, dim=None, axis=None, **kwargs)
    Reduce this DataArray's data by applying `count` along some dimension(s).

    Parameters
    ----------
    dim : str or sequence of str, optional
        Dimension(s) over which to apply `count`.
    axis : int or sequence of int, optional
        Axis(es) over which to apply `count`. Only one of the 'dim'
        and 'axis' arguments can be supplied. If neither are supplied, then
        `count` is calculated over axes.
    keep_attrs : bool, optional
        If True, the attributes (`attrs`) will be copied from the original
        object to the new one.  If False (default), the new object will be
        returned without attributes.
    **kwargs : dict
        Additional keyword arguments passed on to the appropriate array
        function for calculating `count` on this object's data.

    Returns
    -------
    reduced : DataArray
        New DataArray object with `count` applied to its data and the
        indicated dimension(s) removed.

[sjvrijn]

Btw, is there a reason why the functional kwarg keep_attrs has an explanation listed, but isn't listed in the signature? I was unable to find how to update that in the docs when working on this PR.

[dcherian]

Thanks @sjvrijn can you merge the master branch to fix the RTD build? Then we can double check the docs.

[sjvrijn]

@dcherian Docs seem to be correct: skipna is removed from DataArray.count, DataArray.any, DataArray.all while still there in e.g. DataArray.sum

[dcherian]

Thanks @sjvrijn

[max-sixty]

Thanks @sjvrijn !