lavc: Document AVFrame av_opt_ptr() relation.

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

libavcodec/avcodec.h

@@ -944,7 +944,10 @@ typedef struct AVPacket {
 /**
  * Audio Video Frame.
  * New fields can be added to the end of AVFRAME with minor version
- * bumps. Removal, reordering and changes to existing fields require
+ * bumps. Similarly fields that are marked as to be only accessed by
+ * av_opt_ptr() can be reordered. This allows 2 forks to add fields
+ * without breaking compatibility with each other.
+ * Removal, reordering and changes in the remaining cases require
  * a major version bump.
  * sizeof(AVFrame) must not be used outside libav*.
  */