GridSpec.subplots, while user-facing, is relatively more low-level
than Figure.subplots, so it doesn't seem worth duplicating the
docstring in both places (I'm fine having it duplicated both in
pyplot.subplots and Figure.subplots are both are very much
user-facing). Less duplication also means fewer chances to forget to
update the docs in all places, as was the case for the remark regarding
set_units (which is already present in the docs of pyplot.subplots,
though).

Also note that a specific check a bit further down can go away once a
deprecation elapses.

PR Summary

PR Checklist

• Has pytest style unit tests (and pytest passes).
• Is Flake 8 compliant (run flake8 on changed files to check).
• New features are documented, with examples if plot related.
• Documentation is sphinx and numpydoc compliant (the docs should build without error).
• Conforms to Matplotlib style conventions (install flake8-docstrings and run flake8 --docstring-convention=all).
• New features have an entry in doc/users/next_whats_new/ (follow instructions in README.rst there).
• API changes documented in doc/api/next_api_changes/ (follow instructions in README.rst there).