include material on async fn in the Rust reference

[nikomatsakis]

This is a "rust-lang/rust" placeholder for rust-lang/reference#634, since that exists in another repo/org.

[nikomatsakis]

I am taking a first pass at this.

[skade]

If I see that right, rust-lang/reference#635 seems reasonably complete and mainly waits for #62149 to be merged before being included in the reference?

[Centril]

@skade Yeah we wait for the stabilization PR to go through before changing the reference (otherwise it won't pass the tests... ^^); the PR is indeed reasonably complete aside from some nits.

[Centril]

Closing this in favor of tracking the remaining reference work in the reference repo.