rustdoc: render generic params & where-clauses of cross-crate assoc tys in impls
We used to only ever render generic parameters & where-clauses of cross-crate associated types when the item was located inside of a trait and we used to just drop them when it was inside of an impl block (trait or inherent).
Fixes#112904.
`@rustbot` label A-cross-crate-reexports