avcodec/utils: document allocation requirement of extradata

libavcodec/avcodec.h

@@ -1617,6 +1617,7 @@ typedef struct AVCodecContext {
      * The allocated memory should be AV_INPUT_BUFFER_PADDING_SIZE bytes larger
      * than extradata_size to avoid problems if it is read with the bitstream reader.
      * The bytewise contents of extradata must not depend on the architecture or CPU endianness.
+     * Must be allocated with the av_malloc() family of functions.
      * - encoding: Set/allocated/freed by libavcodec.
      * - decoding: Set/allocated/freed by user.
      */