vp8cx.h: add brief desc. for VP9E_SET_SVC_LAYER_ID

fixes doxygen warning related to vpx_svc_layer_id

Change-Id: I365688133c1598182488501158de6af4b0f4c546

vpx/vp8cx.h

@@ -195,6 +195,11 @@ enum vp8e_enc_control_id {
 
   VP9E_SET_SVC,
   VP9E_SET_SVC_PARAMETERS,
+  /*!\brief control function to set svc layer for spatial and temporal.
+   * \note Valid ranges: 0..#vpx_codec_enc_cfg::ss_number_layers for spatial
+   *                     layer and 0..#vpx_codec_enc_cfg::ts_number_layers for
+   *                     temporal layer.
+   */
   VP9E_SET_SVC_LAYER_ID
 };