Document the frames returned by the decoder better.

Originally committed as revision 23456 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavcodec/avcodec.h

@@ -3501,6 +3501,11 @@ attribute_deprecated int avcodec_decode_video(AVCodecContext *avctx, AVFrame *pi
  * @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
  *             allocate memory for the actual bitmap.
+ *             with default get/release_buffer(), the decoder frees/reuses the bitmap as it sees fit.
+ *             with overridden get/release_buffer() the user decides into what buffer the decoder
+ *                   decodes and the decoder tells the user once it does not need the data anymore,
+ *                   the user app can at this point free/reuse/keep the memory as it sees fit.
+ *
  * @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