Add documentation for irreducible metadata

[contradict]

Checklist

• Appropriate tests were added
• Any code changes were done in a way that does not break public API
• All documentation related to code changes were updated
• The new code follows the
  contributor guidelines, in particular the SciML Style Guide and
  COLPRAC.
• Any new documentation only uses public API

Additional context

This is a useful piece of metadata that I have been advised to use in the forums but didn't have any documentation, hopefully this is a good start.