Document auto traits implemented by the return types of `async fn`

[Aaron1011]

Currently, Rustdoc documents auto-trait impls for types, displaying them along with manually written impls. However, there is currently no way of telling what auto traits are implemented by the impl Future type returned by an async fn.

I'm not quite sure what the best way of displaying this is. We need to make it clear to users that the auto trait impls apply to opaque return type - not to the function itself, or to the underlying type returned by the future (e.g. the u8 in async fn foo() -> u8).

[nikomatsakis]

It would be nice if whatever approach we use here also applies to impl Trait return types in general.

[jyn514]

cc @Darksonn , you were asking for something like this in rust-lang/rfcs#2963 (comment).

In retrospect I think this should be possible as long as there aren't name resolution errors or type errors in the body. That may work for tokio but it will definitely break for async-std (#75100).