Update documentation for path argument parsing

[michael-hoss]

Clarify CLI syntax for path arguments and usage.

What does this PR do?

Add details to the documentation about parsing lists of paths. See related issue #802

Before submitting

• Did you read the contributing guideline?
• Did you update the documentation? (readme and public docstrings)
• Did you write unit tests such that there is 100% coverage on related code? (required for bug fixes and new features)
• Did you verify that new and existing tests pass locally?
• Did you make sure that all changes preserve backward compatibility?
• Did you update the CHANGELOG including a pull request link? (not for typos, docs, test updates, or minor internal changes/refactors)

[mauvilsa]

Thank you for contributing!

I will merge this and probably do more changes later.