avformat/avformat: Clarify documentation of av_interleaved_write_frame()

The earlier documentation claimed that av_interleaved_write_frame()
always orders by dts, which is not necessarily true when using muxers
with custom interleavement functions or the audio_preload option.

Furthermore, the documentation stated that libavformat takes ownership
of the reference of the provided packet (if it is refcounted) and that
the caller may not access the data through this reference after the
function returns. This suggests that the returned packet is not blank,
but instead still contains some set, but invalid fields, which implies
that it would be dangerous to unreference this packet again.

But this is not true: av_interleaved_write_frame()'s actual behaviour
is to always output blank packet (even on error). This commit documents
this fact so that callers know that they can directly reuse this packet.

Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>

libavformat/avformat.h

@@ -2248,7 +2248,7 @@ int av_write_frame(AVFormatContext *s, AVPacket *pkt);
  * Write a packet to an output media file ensuring correct interleaving.
  *
  * This function will buffer the packets internally as needed to make sure the
- * packets in the output file are properly interleaved in the order of
+ * packets in the output file are properly interleaved, usually ordered by
  * increasing dts. Callers doing their own interleaving should call
  * av_write_frame() instead of this function.
  *
@@ -2261,10 +2261,10 @@ int av_write_frame(AVFormatContext *s, AVPacket *pkt);
  *            <br>
  *            If the packet is reference-counted, this function will take
  *            ownership of this reference and unreference it later when it sees
- *            fit.
- *            The caller must not access the data through this reference after
- *            this function returns. If the packet is not reference-counted,
- *            libavformat will make a copy.
+ *            fit. If the packet is not reference-counted, libavformat will
+ *            make a copy.
+ *            The returned packet will be blank (as if returned from
+ *            av_packet_alloc()), even on error.
  *            <br>
  *            This parameter can be NULL (at any time, not just at the end), to
  *            flush the interleaving queues.
@@ -2280,10 +2280,9 @@ int av_write_frame(AVFormatContext *s, AVPacket *pkt);
  *            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.
+ *            @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.
+ * @return 0 on success, a negative AVERROR on error.
  *
  * @see av_write_frame(), AVFormatContext.max_interleave_delta
  */