DOC: Figure.subplot: Improve docstring for "autolabel"

[yvonnefroehlich]

Description of proposed changes

This PR aims to improve the docstring of the autolabel parameter of the method Figure.subplot.

Preview:

• https://pygmt-dev--4166.org.readthedocs.build/en/4166/api/generated/pygmt.Figure.subplot.html

Reminders

• Run make format and make check to make sure the code follows the style guide.
• Add tests for new features or tests that would have caught the bug that you're fixing.
• Add new public functions/methods/classes to doc/api/index.rst.
• Write detailed docstrings for all functions/methods.
• If wrapping a new module, open a 'Wrap new GMT module' issue and submit reasonably-sized PRs.
• If adding new functionality, add an example to docstrings or tutorials.

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• /format: automatically format and lint the code