mention that start_time maybe AV_NOPTS_VALUE (from patch by neilb suse de)

Originally committed as revision 10135 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavformat/avformat.h

@@ -312,6 +312,7 @@ typedef struct AVStream {
      * decoding: pts of the first frame of the stream, in stream time base.
      * only set this if you are absolutely 100% sure that the value you set
      * it to really is the pts of the first frame
+     * This may be undefined (AV_NOPTS_VALUE).
      * @note the ASF header does NOT contain a correct start_time the ASF
      * demuxer must NOT set this
      */