Document guarantee that after deregister, token is gone
Following discussion on issue document that after an explicit
deregister, token will not be returned again.  (Important if using a
pointer as a Token.)
@@ -881,7 +881,11 @@ impl Poll {
/// When an `Evented` handle is deregistered, the `Poll` instance will
/// no longer monitor it for readiness state changes. Unlike disabling
/// handles with oneshot, deregistering clears up any internal resources
- /// needed to track the handle.
+ /// needed to track the handle. After an explicit call to this
+ /// method completes, it is guaranteed that the token previously
+ /// registered to this handle will not be returned by a future
+ /// poll, so long as a happens-before relationship is established
+ /// between this call and the poll.
///
/// A handle can be passed back to `register` after it has been
/// deregistered; however, it must be passed back to the **same** `Poll`
