DOC: add_child_axes to axes_api.rst

[jklymak]

PR Summary

As above, the doctoring for add_child_axes wasn't in the docs. OTOH I can see an argument for it not to be, i.e. not private, but not so public...

PR Checklist

• Has Pytest style unit tests
• Code is Flake 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

[ImportanceOfBeingErnest]

I think all methods should be documented. If there is reason for the user not to use a method, its docstrings should simply state it like this ("Note that this method should only be used in rare cases. To create schalala in wololo, use the .schalawolo method instead.")

I would hence approve this (modulo doc tests passing).