rustdoc: List functions that return the documented type

[sbward]

It would be very useful to see all the functions which return a given type A on type A's rustdoc page.

Example: At the time of this posting, std::slice::IterMut is returned by the function fn iter_mut(&mut self) -> IterMut<T>; in the trait std::slice::SliceExt. As a consequence of this relationship, the documentation for std::slice::IterMut, should include a link to std::slice::SliceExt.

[tomjakubowski]

I think this is a dupe, in spirit, of the "search rustdoc by type (a la Hoogle)" issue which I can't find right now on mobile

[alexcrichton]

I agree with @tomjakubowski, closing as a dupe of #12866.