Clarify how allocation works for the picture argument for

avcodec_decode_video3.

Originally committed as revision 23009 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavcodec/avcodec.h

@@ -3464,6 +3464,8 @@ 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.
+ *             Use avcodec_alloc_frame to get an AVFrame, the codec will call
+ *             get_buffer to get memory for the actual image data.
  * @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