Clarify (re)get_buffer() and CODEC_CAP_DR1 relation.

Originally committed as revision 19132 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavcodec/avcodec.h

@@ -1331,6 +1331,9 @@ typedef struct AVCodecContext {
      * If pic.reference is set then the frame will be read later by libavcodec.
      * avcodec_align_dimensions() should be used to find the required width and
      * height, as they normally need to be rounded up to the next multiple of 16.
+     * if CODEC_CAP_DR1 is not set then get_buffer() must call
+     * avcodec_default_get_buffer() instead of providing buffers allocated by
+     * some other means.
      * - encoding: unused
      * - decoding: Set by libavcodec., user can override.
      */
@@ -1959,6 +1962,9 @@ typedef struct AVCodecContext {
      * libavcodec will pass previous buffer in pic, function should return
      * same buffer or new buffer with old frame "painted" into it.
      * If pic.data[0] == NULL must behave like get_buffer().
+     * if CODEC_CAP_DR1 is not set then reget_buffer() must call
+     * avcodec_default_reget_buffer() instead of providing buffers allocated by
+     * some other means.
      * - encoding: unused
      * - decoding: Set by libavcodec., user can override
      */