doc/ffmpeg: Document frame_drop_threshold

doc/ffmpeg.texi

@@ -994,6 +994,13 @@ With -map you can select from which stream the timestamps should be
 taken. You can leave either video or audio unchanged and sync the
 remaining stream(s) to the unchanged one.
 
+@item -frame_drop_threshold @var{parameter}
+Frame drop threshold, which specifies how much behind video frames can
+be before they are dropped. In frame rate units, so 1.0 is one frame.
+The default is -1.1. One possible usecase is to avoid framedrops in case
+of noisy timestamps or to increase frame drop precision in case of exact
+timestamps.
+
 @item -async @var{samples_per_second}
 Audio sync method. "Stretches/squeezes" the audio stream to match the timestamps,
 the parameter is the maximum samples per second by which the audio is changed.