lavf: document that passing valid timestamps to muxers is now mandatory

Also, extend the documentation about setting timestamps.

libavformat/avformat.h

@@ -1629,10 +1629,17 @@ int avformat_write_header(AVFormatContext *s, AVDictionary **options);
  *            <br>
  *            Packet's @ref AVPacket.stream_index "stream_index" field must be
  *            set to the index of the corresponding stream in @ref
- *            AVFormatContext.streams "s->streams". It is very strongly
- *            recommended that timing information (@ref AVPacket.pts "pts", @ref
- *            AVPacket.dts "dts", @ref AVPacket.duration "duration") is set to
- *            correct values.
+ *            AVFormatContext.streams "s->streams".
+ *            <br>
+ *            The timestamps (@ref AVPacket.pts "pts", @ref AVPacket.dts "dts")
+ *            must be set to correct values in the stream's timebase (unless the
+ *            output format is flagged with the AVFMT_NOTIMESTAMPS flag, then
+ *            they can be set to AV_NOPTS_VALUE).
+ *            The dts for subsequent packets passed to this function must be strictly
+ *            increasing when compared in their respective timebases (unless the
+ *            output format is flagged with the AVFMT_TS_NONSTRICT, then they
+ *            merely have to be nondecreasing).  @ref AVPacket.duration
+ *            "duration") should also be set if known.
  * @return < 0 on error, = 0 if OK, 1 if flushed and there is no more data to flush
  *
  * @see av_interleaved_write_frame()
@@ -1662,10 +1669,16 @@ int av_write_frame(AVFormatContext *s, AVPacket *pkt);
  *            <br>
  *            Packet's @ref AVPacket.stream_index "stream_index" field must be
  *            set to the index of the corresponding stream in @ref
- *            AVFormatContext.streams "s->streams". It is very strongly
- *            recommended that timing information (@ref AVPacket.pts "pts", @ref
- *            AVPacket.dts "dts", @ref AVPacket.duration "duration") is set to
- *            correct values.
+ *            AVFormatContext.streams "s->streams".
+ *            <br>
+ *            The timestamps (@ref AVPacket.pts "pts", @ref AVPacket.dts "dts")
+ *            must be set to correct values in the stream's timebase (unless the
+ *            output format is flagged with the AVFMT_NOTIMESTAMPS flag, then
+ *            they can be set to AV_NOPTS_VALUE).
+ *            The dts for subsequent packets in one stream must be strictly
+ *            increasing (unless the output format is flagged with the
+ *            AVFMT_TS_NONSTRICT, then they merely have to be nondecreasing).
+ *            @ref AVPacket.duration "duration") should also be set if known.
  *
  * @return 0 on success, a negative AVERROR on error. Libavformat will always
  *         take care of freeing the packet, even if this function fails.