Clarify ref_index.

Originally committed as revision 22068 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavcodec/avcodec.h

@@ -894,6 +894,7 @@ typedef struct AVPanScan{
 \
     /**\
      * motion referece frame index\
+     * the order in which these are stored can depends on the codec.\
      * - encoding: Set by user.\
      * - decoding: Set by libavcodec.\
      */\