Add links and expand mathmpl docstring

[melissawm]

PR Summary

This PR clarifies the purpose and recommended uses for mathmpl and adds links to the relevant information in the tutorial and "Writing documentation" pages.

PR Checklist

Documentation and Tests

• [N/A] Has pytest style unit tests (and pytest passes)
• Documentation is sphinx and numpydoc compliant (the docs should build without error).
• [N/A] New plotting related features are documented with examples.

Release Notes

• [N/A] New features are marked with a .. versionadded:: directive in the docstring and documented in doc/users/next_whats_new/
• [N/A] API changes are marked with a .. versionchanged:: directive in the docstring and documented in doc/api/next_api_changes/
• [N/A] Release notes conform with instructions in next_whats_new/README.rst or next_api_changes/README.rst

[story645]

GitHub needs a more active status than 'leave a comment' which is kinda passive aggressive...or a fine tuned what is the blocking request.

Most of these are flow suggestions and the rest are changes to language that was already there but I figure should be changed to achieve the aims of this PR but I can open the PR if out of scope.

[story645]

I don't even know if I'm being nitpicky here. Can take or leave my comments, just let me know so I can merge either way.

[melissawm]

Thanks for the comments - they do help! Don't forget also I'm not a native speaker so rewording suggestions are always welcome 😄

[QuLogic]

Should wrap.

[melissawm]

Thanks @QuLogic - I think I got everything and also rebased just in case.

[QuLogic]

Not sure why the errors didn't post:

/home/circleci/project/doc/devel/documenting_mpl.rst:352: ERROR: Unexpected indentation.
/home/circleci/project/doc/devel/documenting_mpl.rst:353: WARNING: Block quote ends without a blank line; unexpected unindent.
looking for now-outdated files... none found
pickling environment... done
checking consistency... done
preparing documents... done
/home/circleci/project/lib/matplotlib/sphinxext/mathmpl.py:docstring of matplotlib.sphinxext.mathmpl:4: WARNING: unknown document: '../tutorials/text/mathtext'
/home/circleci/project/doc/devel/documenting_mpl.rst:353: WARNING: unknown document: '../tutorials/text/mathtext'

[melissawm]

I think that did it!