Clarify section about associated types being reserved

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.)

Author: traviscross

text/3668-async-closure.md

@@ -193,7 +193,34 @@ pub trait AsyncFn<Args>: AsyncFnMut<Args> {
 
 ### Associated types of `AsyncFn*` traits are not nameable
 
-Note that, unlike what is true today with the current `Fn*` traits, this RFC reserves as an implementation detail the associates types of the `AsyncFn*` traits, and these will not be nameable as part of the stable interface specified by this RFC.
+Unlike what is true today with the current `Fn*` traits, this RFC reserves as an implementation detail the associated types of the `AsyncFn*` traits, and these will not be nameable as part of the stable interface specified by this RFC.
+
+That is, using the existing `FnOnce` trait, we can write this today on stable Rust:
+
+```rust
+fn foo<F, T>()
+where
+    F: FnOnce() -> T,
+    F::Output: Send, //~ OK
+{
+}
+```
+
+(We decided to allow this in [#34365](https://github.com/rust-lang/rust/pull/34365).)
+
+However, this RFC reserves as an implementation detail the associated types of the traits specified above, so this does not work:
+
+```rust
+fn foo<F, T>()
+where
+    F: async FnOnce() -> T,
+    F::Output: Send,
+    //~^ ERROR use of unstable library feature
+    F::CallOnceFuture: Send,
+    //~^ ERROR use of unstable library feature
+{
+}
+```
 
 ### `async` bound modifier on `Fn()` trait bounds