Add multiframe note to avcodec_decode_video() too.

Originally committed as revision 18702 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavcodec/avcodec.h

@@ -3126,6 +3126,8 @@ attribute_deprecated int avcodec_decode_video(AVCodecContext *avctx, AVFrame *pi
 
 /**
  * Decodes the video frame of size avpkt->size from avpkt->data into picture.
+ * Some decoders may support multiple frames in a single AVPacket, such
+ * decoders would then just decode the first frame.
  *
  * @warning The input buffer must be \c FF_INPUT_BUFFER_PADDING_SIZE larger than
  * the actual read bytes because some optimized bitstream readers read 32 or 64