REF: avoid overriding some NDFrame methods
The only change in most places is that the docstrings say "Series/DataFrame" instead of just "Series" for the Series method and "DataFrame" in the DataFrame method. 

In fillna the axis keyword says "{0 or 'index'} for Series, {0 or 'index', 1 or 'columns'} for DataFrame" instead of being more specific in each subclass.  Same for fill_axis and broadcast_axis (both of which are deprecated) in align
Thanks @jbrockmendel 