DOC/Projections "Polar": General improvements

[yvonnefroehlich]

Description of proposed changes

This PR contains general improvements of the Projection example "Polar":

• Make argument passed to xshiftand yshift flexible
• Re-wrapp to 88 chars

Preview:

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