Add more context when trying to write async trait function

[estebank]

Given

trait T {
    async fn foo() {}
    async fn bar(&self) {}
}

we currently emit

error[E0706]: trait fns cannot be declared `async`
 --> src/lib.rs:2:5
  |
2 |     async fn foo() {}
  |     ^^^^^^^^^^^^^^^^^

error[E0706]: trait fns cannot be declared `async`
 --> src/lib.rs:3:5
  |
3 |     async fn bar(&self) {}
  |     ^^^^^^^^^^^^^^^^^^^^^^

We should tell people that async trait functions are not possible because of technical restrictions, and possibly point them to the async-trait crate.

This issue has been assigned to @jafern14 via this comment.

[jafern14]

@rustbot claim

[jafern14]

Created #65937