Docstring contains details specific to RectangleSelector and does not fully generalize to EllipseSelector.

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   PR Summary

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Matplotlib documentation is missing details concerning the geometry attribute for RectangleSelector.
This change provides a docstring for said attribute which is specific to RectangleSelector and
not generalizable to EllipseSelector.
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   PR Checklist

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

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