lavfi: document that AVFilterLink is not for applications.

libavfilter/avfilter.h

@@ -383,6 +383,11 @@ struct AVFilterContext {
  * the pads involved. In addition, this link also contains the parameters
  * which have been negotiated and agreed upon between the filter, such as
  * image dimensions, format, etc.
+ *
+ * Applications must not normally access the link structure directly.
+ * Use the buffersrc and buffersink API instead.
+ * In the future, access to the header may be reserved for filters
+ * implementation.
  */
 struct AVFilterLink {
     AVFilterContext *src;       ///< source filter