Try to improve reordered_opaque doxy.

Originally committed as revision 25876 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavcodec/avcodec.h

@@ -992,8 +992,12 @@ typedef struct AVPanScan{
     int8_t *ref_index[2];\
 \
     /**\
-     * reordered opaque 64bit number (generally a PTS) from AVCodecContext.reordered_opaque\
-     * output in AVFrame.reordered_opaque\
+     * reordered opaque 64bit (generally a integer or double preccission float \
+     * PTS but can be anything). \
+     * The user sets AVCodecContext.reordered_opaque to represent the input at\
+     * that time ,\
+     * the decoder reorderes values as needed and sets AVFrame.reordered_opaque\
+     * to exactly one of the values provided by the user through AVCodecContext.reordered_opaque \
      * - encoding: unused\
      * - decoding: Read by user.\
      */\