More complete documentation for AVFrame.reference.

Originally committed as revision 12518 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavcodec/avcodec.h

@@ -601,6 +601,8 @@ typedef struct AVPanScan{
 \
     /**\
      * is this picture used as reference\
+     * The values for this are the same as the MpegEncContext.picture_structure\
+     * variable, that is 1->top field, 2->bottom field, 3->frame/both fields.\
      * - encoding: unused\
      * - decoding: Set by libavcodec. (before get_buffer() call)).\
      */\