Add comments for two SVC related controls

VP9E_SET_SVC
VP9E_SET_SVC_PARAMETERS

Change-Id: Ie32ffad48b6b53c5a2c44c42ec655bfedf8d2903

vpx/vp8cx.h

diff --git a/vpx/vp8cx.h b/vpx/vp8cx.h
index fbf82c2..be6e9fe 100644
--- a/vpx/vp8cx.h
+++ b/vpx/vp8cx.h
@@ -328,7 +328,17 @@
    */
   VP9E_SET_NOISE_SENSITIVITY,
 
+  /*!\brief control function to turn on/off SVC in encoder.
+   * \note Return value is VPX_CODEC_INVALID_PARAM if the encoder does not
+   *       support SVC in its current encoding mode
+   *  0: off, 1: on
+   */
   VP9E_SET_SVC,
+
+  /*!\brief control function to set parameters for SVC.
+   * \note Parameters contain min_q, max_q, scaling factor for each of the
+   *       SVC layers.
+   */
   VP9E_SET_SVC_PARAMETERS,
 
   /*!\brief control function to set svc layer for spatial and temporal.