We specify that the details about the traits, including the associated
types, are implementation details.  Let's make the section describing
this, with respect to the associated types, more clear and give an
example.

(Thanks to RalfJ and Yosh for raising that this section could be
clarified.)

Co-authored-by: Yosh <github@yosh.is>

whoops forgot a mut

Co-authored-by: zachs18 <8355914+zachs18@users.noreply.github.com>

The feature gate for async closures is unfortunately in the singular,
`async_closure`, and while that's not a stable commitment, it's
probably too much of a pain to change that.  But that doesn't mean
that we can't name the file in the plural, so let's do that.

As it turns out, `AsyncFn()` probably commits our direction on
generalized `async` trait bound modifiers to the same degree as `async
Fn()`.  Let's describe this.

CE brought up this excellent point.

add the note in one other place

Co-authored-by: Josh Triplett <josh@joshtriplett.org>