DOC/Gallery example "Cross-section along a transect": General improvements

[yvonnefroehlich]

Description of proposed changes

This PR contains general improvements of the Gallery example "Cross-section along a transect":

• Use lists as input for the center and endpoint parameters
• Make argument passed to xshift flexible
• Re-wrapp to 88 chars
• Improve comments

Preview: https://pygmt-dev--3623.org.readthedocs.build/en/3623/gallery/images/cross_section.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 command is:

• /format: automatically format and lint the code

[seisman]

The changes look good to me. Could you please also rewrap the lines to <88 characters?

[yvonnefroehlich]

The changes look good to me. Could you please also rewrap the lines to <88 characters?

Done in afbbdab.
Edit: and df1a8de.