lavc: Extend the documentation for avcodec_init_packet

Add a note that pkt->data and pkt->size must be initialized.
Signed-off-by: Martin Storsjö <martin@martin.st>

libavcodec/avcodec.h

@@ -3284,6 +3284,9 @@ void av_destruct_packet(AVPacket *pkt);
 /**
  * Initialize optional fields of a packet with default values.
  *
+ * Note, this does not touch the data and size members, which have to be
+ * initialized separately.
+ *
  * @param pkt packet
  */
 void av_init_packet(AVPacket *pkt);