DOC: clarify GridSpec ratio updates - #32415

PR summary

GridSpec.set_width_ratios and set_height_ratios update the ratios stored by a GridSpec, but existing Axes are not repositioned unless a layout engine runs or their SubplotSpec is re-evaluated. This behavior is currently implicit and can be surprising when a setter is called after Axes have been created.

This change documents that existing-Axes behavior in both setter docstrings and points users to the supported ways to apply the updated ratios.

Closes #31971

AI Disclosure

I used generative AI to help inspect the repository, identify the relevant issue, and proofread this description. I reviewed and verified the change and the test result myself.

PR quality check

• Use an expressive title
• New and changed code is tested (documentation-only change; existing GridSpec tests pass)
• Plotting related features are demonstrated in an example (not applicable)
• New features and API changes have release notes (not applicable; documentation clarification only)
• Documentation complies with the contribution guidelines

Testing

• ./.venv/bin/pytest -q lib/matplotlib/tests/test_gridspec.py (7 passed)
• git diff --check