avcodec: doxy: Clarify what get_buffer2 must fill in buf[]

Signed-off-by: Derek Buitenhuis <derek.buitenhuis@gmail.com>

libavcodec/avcodec.h

@@ -1920,8 +1920,11 @@ typedef struct AVCodecContext {
      *     extended_data must be allocated with av_malloc() and will be freed in
      *     av_frame_unref().
      *   * otherwise exended_data must point to data
-     * - buf[] must contain references to the buffers that contain the frame
-     *   data.
+     * - buf[] must contain one or more pointers to AVBufferRef structures. Each of
+     *   the frame's data and extended_data pointers must be contained in these. That
+     *   is, one AVBufferRef for each allocated chunk of memory, not necessarily one
+     *   AVBufferRef per data[] entry. See: av_buffer_create(), av_buffer_alloc(),
+     *   and av_buffer_ref().
      * - extended_buf and nb_extended_buf must be allocated with av_malloc() by
      *   this callback and filled with the extra buffers if there are more
      *   buffers than buf[] can hold. extended_buf will be freed in