avcodec/pthread_frame: update doxygen for update_context_from_thread

Signed-off-by: Ganesh Ajjanagadde <gajjanagadde@gmail.com>
Signed-off-by: Ronald S. Bultje <rsbultje@gmail.com>

libavcodec/pthread_frame.c

@@ -187,6 +187,7 @@ static attribute_align_arg void *frame_worker_thread(void *arg)
  * @param dst The destination context.
  * @param src The source context.
  * @param for_user 0 if the destination is a codec thread, 1 if the destination is the user's thread
+ * @return 0 on success, negative error code on failure
  */
 static int update_context_from_thread(AVCodecContext *dst, AVCodecContext *src, int for_user)
 {