add document.

src/ipc.c

@@ -1086,7 +1086,7 @@ RTM_EXPORT(rt_event_send);
  * @param option the receive option, either RT_EVENT_FLAG_AND or
  *        RT_EVENT_FLAG_OR should be set.
  * @param timeout the waiting time
- * @param recved the received event
+ * @param recved the received event, if you don't care, RT_NULL can be set.
  *
  * @return the error code
  */