message: Add improve FB doc : add handover request thread control
diff: @@ -637,6 +637,18 @@ controller.api.handover.pass_thread_control('<RECIPIENT_PSID>', '<TARGET_PSID>',
  });
  ```
  +### Request Thread Control
  +
  +The Request Thread Control API allows a Secondary Receiver app to notify the Primary Receiver that it wants control of the chat :
  +
  +- To pass thread control:
  +```javascript
  +controller.api.handover.request_thread_control('<RECIPIENT_PSID>', 'String to pass to request the thread control', function (result) {
  +
  +});
  +```
  +
  +
  ## Messaging type
  You can identify the purpose of the message being sent to Facebook by adding `messaging_type: <MESSAGING_TYPE>` property when sending the message:
  