DOC: use _shared_docs for fillna methods
currently the docstring for `DataFrame.fillna` is shown for `Series.fillna` and `Panel.fillna`. http://pandas.pydata.org/pandas-docs/version/0.16.0/generated/pandas.Panel.fillna.html?highlight=fillna#pandas.Panel.fillna 

this is not accurate in particular for the `axis` values. This PR makes it such that they share the same template with different `axis` descriptions. 

@sinhrks updated according to your comments 
 @mortada thanks you sir!
