Document async block types

[kpreid]

The section Types lists most kinds of types that can exist in a Rust program, but it does not list the types introduced by async {} and async fn. These types should be documented, since like closure types, they have nontrivial properties such as which traits they implement.

It's also unclear (at least to me) what they should be called, formally and generally — async block types? generated Future types? coroutine types? — and having that terminology nailed down would help teach Rust by avoiding inconsistent or ambiguous terminology.

@rustbot label A-async "New Content"