Extend selectors documentation with properties

[jhrozek]

Summary

Since we can now use properties in selectors, let's document the
available properties.

Change Type

• Bug fix (resolves an issue without affecting existing features)
• Feature (adds new functionality without breaking changes)
• Breaking change (may impact existing functionalities or require documentation updates)
• Documentation (updates or additions to documentation)
• Refactoring or test improvements (no bug fixes or new functionality)

Testing

N/A

Review Checklist:

• Reviewed my own code for quality and clarity.
• Added comments to complex or tricky code sections.
• Updated any affected documentation.
• Included tests that validate the fix or feature.
• Checked that related changes are merged.

[coveralls]

coverage: 53.73% (+0.008%) from 53.722%
when pulling 14b3758 on jhrozek:selector_docs
into 614ba33 on stacklok:main.