message: Improve doc : Add cancel scheduled broadcast section
diff: @@ -649,6 +649,15 @@ controller.api.handover.request_thread_control('<RECIPIENT_PSID>', 'String to pa
  });
  ```
  +### Cancelling a Scheduled Broadcast
  +
  +- To cancel a scheduled broadcast :
  +```javascript
  +controller.api.handover.cancel_scheduled_broadcast('<BROADCAST_ID>', function (result) {
  +
  +});
  +```
  +
  ## Messaging type
  