User's Guide: document DefaultSignatures' interaction with subsumption

As reported in #19432 (closed), the rules governing how DefaultSignatures are typechecked became stricter in GHC 9.0 due to simplified subsumption. However, this was far from obvious to me after reading the User's Guide section on DefaultSignatures. In this patch, I spruce up the documentation in that section so that it mentions these nuances.

Resolves #19432 (closed).