Add comments for two SVC related controls

VP9E_GET_SVC_LAYER_ID
VP9E_REGISTER_CX_CALLBACK

Change-Id: I92060bde1a4830268c8e07190d4ee4c37bc05334

vpx/vp8cx.h

@@ -344,7 +344,17 @@ enum vp8e_enc_control_id {
    *              VP9E_CONTENT_SCREEN  = Screen capture content
    */
   VP9E_SET_TUNE_CONTENT,
+
+  /*!\brief control function to get svc layer ID.
+   * \note The layer ID returned is for the data packet from the registered
+   *       callback function.
+   */
   VP9E_GET_SVC_LAYER_ID,
+
+  /*!\brief control function to register callback for getting per layer packet.
+   * \note Parameter for this control function is a structure with a callback
+   *       function and a pointer to private data used by the callback.
+   */
   VP9E_REGISTER_CX_CALLBACK,
 };