Rustdoc should show associated types for an impl under "implementors"

[Manishearth]

Currently rustdoc doesn't show associated types under the "implementors" portion of the trait page (eg here) (though it does show them on the page for the type). Since associated types are as important as type parameters when inspecting an impl (and debugging a type mismatch), perhaps rustdoc should show an (Output = [T]) or something similar next to each impl?

In case of a cross crate impl where the trait shares a crate with the impl, the associated type will be missing from rustdoc altogether.

[frewsxcv]

This is probably a duplicate of #21092