lavf: clarify the lifetime of demuxed packets.

libavformat/avformat.h

@@ -158,9 +158,15 @@
  * information will be in AVStream.time_base units, i.e. it has to be
  * multiplied by the timebase to convert them to seconds.
  *
- * The packet data belongs to the demuxer and is invalid after the next call to
- * av_read_frame(). The user must free the packet with av_free_packet() before
- * calling av_read_frame() again or closing the file.
+ * If AVPacket.destruct is set on the returned packet, then the packet is
+ * allocated dynamically and the user may keep it indefinitely.
+ * Otherwise, if AVPacket.destruct is NULL, the packet data is backed by a
+ * static storage somewhere inside the demuxer and the packet is only valid
+ * until the next av_read_frame() call or closing the file. If the caller
+ * requires a longer lifetime, av_dup_packet() will make an av_malloc()ed copy
+ * of it.
+ * In both cases, the packet must be freed with av_free_packet() when it is no
+ * longer needed.
  *
  * @section lavf_decoding_seek Seeking
  * @}
@@ -1307,13 +1313,13 @@ int av_read_packet(AVFormatContext *s, AVPacket *pkt);
  * omit invalid data between valid frames so as to give the decoder the maximum
  * information possible for decoding.
  *
- * The returned packet is valid
- * until the next av_read_frame() or until av_close_input_file() and
- * must be freed with av_free_packet. For video, the packet contains
- * exactly one frame. For audio, it contains an integer number of
- * frames if each frame has a known fixed size (e.g. PCM or ADPCM
- * data). If the audio frames have a variable size (e.g. MPEG audio),
- * then it contains one frame.
+ * If pkt->destruct is NULL, then the packet is valid until the next
+ * av_read_frame() or until av_close_input_file(). Otherwise the packet is valid
+ * indefinitely. In both cases the packet must be freed with
+ * av_free_packet when it is no longer needed. For video, the packet contains
+ * exactly one frame. For audio, it contains an integer number of frames if each
+ * frame has a known fixed size (e.g. PCM or ADPCM data). If the audio frames
+ * have a variable size (e.g. MPEG audio), then it contains one frame.
  *
  * pkt->pts, pkt->dts and pkt->duration are always set to correct
  * values in AVStream.time_base units (and guessed if the format cannot