Figure.velo: Update docstrings related to "fill" and "uncertaintyfill"

[yvonnefroehlich]

Description of proposed changes

This PR aims to update the docstrings related to "fill" and "uncertaintyfill". It removes the term "shade" and adds the term "pattern". Related to

• the disccssion in Figure.meca: Add aliases for C, E, G, and W #2345 (comment)
• PR Improve docstrings for "fill" #2351

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