Minor fixes to async notes (#607)

* `join!( .., time::sleep(..))` isn't really a "timeout"

* Remove suggestion to make `sleep` take `&mut self`

I'm not sure what doing so demonstrates.

* Update src/async/control-flow/join.md

Co-authored-by: rbehjati <razieh@google.com>

---------

Co-authored-by: rbehjati <razieh@google.com>

src/async/control-flow/join.md

@@ -43,9 +43,8 @@ Copy this example into your prepared `src/main.rs` and run it from there.
   cause your program to stall. 
 
 * You can also combine `join_all` with `join!` for instance to join all requests
-  to an http service as well as a database query.
-
-* Try adding a timeout to the future, using `futures::join!`.
+  to an http service as well as a database query. Try adding a
+  `tokio::time::sleep` to the future, using `futures::join!`. This is not a
+  timeout (that requires `select!`, explained in the next chapter), but demonstrates `join!`.
 
 </details>
-

src/async/pitfalls/async-traits.md

@@ -60,6 +60,4 @@ async fn main() {
 * Try creating a new sleeper struct that will sleep for a random amount of time
   and adding it to the Vec.
 
-* Try making the `sleep` call take `&mut self`.
-
 </details>