Correct way to document implementations of methods in new style

[PokhodenkoSA]

Actually axis could be a string ('index' or 'columns'), not only an integer. You could add string type here. Moreover you could check on typing stage of the implementation that axis type is able to be a string.

Originally posted by @densmirn in #227

[PokhodenkoSA]

Pandas docs: {index (0)}
So it supports integers and columns and names.
I prefer copy docs from Pandas and add flags like unsupported to indicate that parameter behave not like in Pandas.