AVFrame.pkt_dts: improve docs

Related to Ticket2375
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavutil/frame.h

@@ -164,7 +164,9 @@ typedef struct AVFrame {
     int64_t pkt_pts;
 
     /**
-     * DTS copied from the AVPacket that triggered returning this frame.
+     * DTS copied from the AVPacket that triggered returning this frame. (if frame threading isnt used)
+     * This is also the Presentation time of this AVFrame calculated from
+     * only AVPacket.dts values without pts values.
      */
     int64_t pkt_dts;