lavc: Document that AVOptions should be used.

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/avcodec.h

@@ -1185,6 +1185,8 @@ typedef struct AVFrame {
  * New fields can be added to the end with minor version bumps.
  * Removal, reordering and changes to existing fields require a major
  * version bump.
+ * Please use AVOptions (av_opt_set/get*()) to access these fields from user
+ * applications.
  * sizeof(AVCodecContext) must not be used outside libav*.
  */
 typedef struct AVCodecContext {