Expand documentation for Cartesian projections

[yvonnefroehlich]

Description of proposed changes

This PR aims to expand the documentations of the Cartesian linear, logarithmic, and power projections regarding the scale option.

Currently, the general syntax is repeated for the scale option (lower-case letter version), as we have it for nearly all other projections. Or do we only want to state the width-height version (upper-case letter version), as is currently done for the polar projection?

For context / first mentioned: #2641 (comment)

Preview:

• Cartesian linear
• Cartesian logarithmic
• Cartesian power

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.

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