Extensions: xml docs and cref follow-ups


- [x] We want unqualified CREF to work within the context of enclosing static type `cref="extension(int).M"` (https://github.com/dotnet/roslyn/pull/80114)
- [ ] Missing warning for duplicated `typeparam` on merged extension (tracked in a test)

Operators
- [x]  https://github.com/dotnet/roslyn/pull/79227 - 'cref' binding

Relates to test plan https://github.com/dotnet/roslyn/issues/76130
