avfilter: update some comments

start_frame is no more, samplesref argument is now frame argument.
Signed-off-by: Paul B Mahol <onemda@gmail.com>

libavfilter/avfilter.h

@@ -491,7 +491,7 @@ struct AVFilterLink {
 
     /**
      * Link status.
-     * If not zero, all attempts of start_frame, filter_frame or request_frame
+     * If not zero, all attempts of filter_frame or request_frame
      * will fail with the corresponding code, and if necessary the reference
      * will be destroyed.
      * If request_frame returns an error, the status is set on the

libavfilter/internal.h

@@ -86,7 +86,7 @@ struct AVFilterPad {
      * Input pads only.
      *
      * @return >= 0 on success, a negative AVERROR on error. This function
-     * must ensure that samplesref is properly unreferenced on error if it
+     * must ensure that frame is properly unreferenced on error if it
      * hasn't been passed on to another filter.
      */
     int (*filter_frame)(AVFilterLink *link, AVFrame *frame);