Clarify the input AVPacket for avcodec_decode*()

Originally committed as revision 19077 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavcodec/avcodec.h

@@ -3215,6 +3215,9 @@ attribute_deprecated int avcodec_decode_audio2(AVCodecContext *avctx, int16_t *s
  * @param[out] samples the output buffer
  * @param[in,out] frame_size_ptr the output buffer size in bytes
  * @param[in] avpkt The input AVPacket containing the input buffer.
+ *            You can create such packet with av_init_packet() and by then setting
+ *            data and size, some decoders might in addition need other fields.
+ *            All decoders are designed to use the least fields possible though.
  * @return On error a negative value is returned, otherwise the number of bytes
  * used or zero if no frame could be decompressed.
  */
@@ -3268,6 +3271,10 @@ attribute_deprecated int avcodec_decode_video(AVCodecContext *avctx, AVFrame *pi
  * @param avctx the codec context
  * @param[out] picture The AVFrame in which the decoded video frame will be stored.
  * @param[in] avpkt The input AVpacket containing the input buffer.
+ *            You can create such packet with av_init_packet() and by then setting
+ *            data and size, some decoders might in addition need other fields like
+ *            flags&PKT_FLAG_KEY. All decoders are designed to use the least
+ *            fields possible.
  * @param[in,out] got_picture_ptr Zero if no frame could be decompressed, otherwise, it is nonzero.
  * @return On error a negative value is returned, otherwise the number of bytes
  * used or zero if no frame could be decompressed.