Extensions: xml docs and cref follow-ups

[jcouv]

• We want unqualified CREF to work within the context of enclosing static type cref="extension(int).M" (Extensions: allow more CREFs #80114)
• Missing warning for duplicated typeparam on merged extension (tracked in a test)

Operators

• Address another set of functionality gaps for extension operators #79227 - 'cref' binding

Relates to test plan #76130