message: Add clarifying comment to cancellation handling in rendezvous.cc.
diff: @@ -266,6 +266,9 @@ class LocalRendezvousImpl : public Rendezvous {
  Item* item = new Item;
  if (cm != nullptr) {
  + // NOTE(mrry): We must wrap `done` with code that deregisters the
  + // cancellation callback before calling the `done` callback, because the
  + // cancellation manager may no longer be live after `done` is called.
  auto wrapped_done = std::bind(
  [cm, token](const DoneCallback& done,
  // Begin unbound arguments.
  