update docs

Author: arch

docs/app/docs/user-guide/config.md

@@ -26,6 +26,7 @@ Config Files:
 - `preview_scaling` (float): Set the preview image scaling factor. With a value of `1.0`, the window should fill the height or width of the screen depending on the aspect ratio of the video.
 - `tracker`: (str) Specify the tracker algorithm. Available options are `'MIL'`, `'KCF'`, `'CSRT'`.
 - `notification_sound` (str) Specify the wav file to play when tracking finished (write 'off' to disable the sound notification).
+- `tracking_lost_time` (int) Time in milliseconds at which the tracking is stopped if the selected feature is not found.
 
 #### `hyperparameter.yaml`