Update region and projection standard docstrings

[willschlitzer]

This pull request modifies the COMMON_OPTIONS text for region and projection. It also adds that it is requirement for certain plotting commands to individual modules. This partially addresses #1493.

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 adding new functionality, add an example to docstrings or tutorials.

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]

The doctest in

pygmt/pygmt/helpers/decorators.py

Line 451 in c81555e

*Required if this is the first plot command*.

fails. You need to remove the text *Required if this is the first plot command*. from the fmt_docstring doctest.

[seisman]

Looks good to me after applying @meghanrjones's suggestions.