Extend description and name in the same way

[hersle]

Small follow-up to #3132. Changes the behavior of extend(sys, basesys) to inherit the description of sys, following the same pattern as name. I think this is the better, most consistent and most flexible behavior.

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