[EventEngine] Update Cancel contract on closure deletion timeline (#34167)

CC @ctiller @Vignesh2208

include/grpc/event_engine/event_engine.h

@@ -318,7 +318,7 @@ class EventEngine : public std::enable_shared_from_this<EventEngine> {
   ///
   /// If the associated connection has not been completed, it will be cancelled,
   /// and this method will return true. The \a OnConnectCallback will not be
-  /// called.
+  /// called, and \a on_connect will be destroyed before this method returns.
   virtual bool CancelConnect(ConnectionHandle handle) = 0;
   /// Provides asynchronous resolution.
   ///
@@ -457,13 +457,9 @@ class EventEngine : public std::enable_shared_from_this<EventEngine> {
   /// be cancelled, and this function will return false.
   ///
   /// If the associated closure has not been scheduled to run, it will be
-  /// cancelled, and the associated absl::AnyInvocable or \a Closure* will not
-  /// be executed. In this case, Cancel will return true.
-  ///
-  /// Implementation note: closures should be destroyed in a timely manner after
-  /// execution or cancellation (milliseconds), since any state bound to the
-  /// closure may need to be destroyed for things to progress (e.g., if a
-  /// closure holds a ref to some ref-counted object).
+  /// cancelled, and this method will return true. The associated
+  /// absl::AnyInvocable or \a Closure* will not be called. If the closure type
+  /// was an absl::AnyInvocable, it will be destroyed before the method returns.
   virtual bool Cancel(TaskHandle handle) = 0;
 };