Converting docstrings to numpydoc #7205

Some of the quite well used methodes of the Axes object haven't been ported to numpy doc yet.
This ticket aims at listing those, and slowly moving those functions to numpy doc.

• set_xticklabels, set_yticklabels (missing documentation of some args)
• Figure.add_subplot, Figure.add_axes
• twinx, twiny
• set_xlim, set_ylim
• set_xlim, set_ylim
• set_xscale, set_yscale

Contributors, feel free to update this list as need be!

Here is some documentation: https://github.com/numpy/numpy/blob/master/doc/HOWTO_DOCUMENT.rst.txt