Figure.meca: Expand docstring for the scale parameter

[yvonnefroehlich]

Description of proposed changes

This PR aims to expand the docstring for the scale parameter of pygmt.Figure.meca.

Upstream GMT documentation: https://docs.generic-mapping-tools.org/dev/supplements/seis/meca.html#s

Preview: https://pygmt-dev--2552.org.readthedocs.build/en/2552/api/generated/pygmt.Figure.meca.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.
• Use underscores (not hyphens) in names of Python files and directories.

Slash Commands

You can write slash commands (/command) in the first line of a comment to perform
specific operations. Supported slash commands are:

• /format: automatically format and lint the code
• /test-gmt-dev: run full tests on the latest GMT development version

[seisman]

Related to #1466, but I'm not sure if we need a more Pythonic solution.

[seisman]

Looks good to me. We can have a more Pythonic Figure.meca function later if we want.

[seisman]

Ping @GenericMappingTools/pygmt-maintainers to review this PR.