clarify convergence_duration documentation

Originally committed as revision 24001 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavcodec/avcodec.h

@@ -995,6 +995,7 @@ typedef struct AVPacket {
      * the very first frame or from this keyframe.
      * Is AV_NOPTS_VALUE if unknown.
      * This field is not the display duration of the current packet.
+     * This field has no meaning if the packet do not have AV_PKT_FLAG_KEY.
      *
      * The purpose of this field is to allow seeking in streams that have no
      * keyframes in the conventional sense. It corresponds to the
@@ -3692,6 +3693,7 @@ typedef struct AVCodecParserContext {
      * the very first frame or from this keyframe.
      * Is AV_NOPTS_VALUE if unknown.
      * This field is not the display duration of the current frame.
+     * This field has no meaning if the packet do not have AV_PKT_FLAG_KEY.
      *
      * The purpose of this field is to allow seeking in streams that have no
      * keyframes in the conventional sense. It corresponds to the