Completed documentation of reference field.

Originally committed as revision 17597 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavcodec/avcodec.h

@@ -658,6 +658,7 @@ 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.\
+     * Set to 4 for delayed, non-reference frames.\
      * - encoding: unused\
      * - decoding: Set by libavcodec. (before get_buffer() call)).\
      */\