extradata endianness comment

Originally committed as revision 4674 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavcodec/avcodec.h

@@ -738,6 +738,7 @@ typedef struct AVCodecContext {
      * mpeg4: global headers (they can be in the bitstream or here)
      * the allocated memory should be FF_INPUT_BUFFER_PADDING_SIZE bytes larger
      * then extradata_size to avoid prolems if its read with the bitstream reader
+     * the bytewise contents of extradata must not depend on the architecture or cpu endianness
      * - encoding: set/allocated/freed by lavc.
      * - decoding: set/allocated/freed by user.
      */