PR Summary

Following #11060, this PR try to improve the documentation of the axes_grid1's inset_axes.

Note that the api of inset_axes is more-or-less inclined to use with the TransformedBbox instance (e.g. ax.bbox, fig.bbox). And a note is added to make this point.

PR Checklist

• Has Pytest style unit tests
• Code is PEP 8 compliant
• New features are documented, with examples if plot related
• Documentation is sphinx and numpydoc compliant
• Added an entry to doc/users/next_whats_new/ if major new feature (follow instructions in README.rst there)
• Documented in doc/api/api_changes.rst if API changed in a backward-incompatible way