Clarify ref_index.

git-svn-id: file:///var/local/repositories/ffmpeg/trunk@22068 9553f0bf-9b14-0410-a0b8-cfaf0461ba5b

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.\
      */\