Document the need of buf=NULL buf_size=0 at the end to obtain all frames.

Originally committed as revision 16482 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavcodec/avcodec.h

@@ -2797,6 +2797,9 @@ int avcodec_decode_audio2(AVCodecContext *avctx, int16_t *samples,
  * the linesize is not a multiple of 16 then there's no sense in aligning the
  * start of the buffer to 16.
  *
+ * @note Some codecs have a delay between input and output, these need to be
+ * feeded with buf=NULL, buf_size=0 at the end to return the remaining frames.
+ *
  * @param avctx the codec context
  * @param[out] picture The AVFrame in which the decoded video frame will be stored.
  * @param[in] buf the input buffer