Add documentation for files_no_background_scan

Signed-off-by: Daniel Kesselberg <mail@danielkesselberg.de>

config/config.sample.php

@@ -1965,4 +1965,15 @@ $CONFIG = [
  */
 
 'login_form_autocomplete' => true,
+
+/**
+ * Disable background scanning of files
+ *
+ * By default, a background job runs every 10 minutes and execute a background
+ * scan to sync filesystem and database. Only users with unscanned files
+ * (size=0 in filecache) are included. Maximum 500 users per job.
+ *
+ * Defaults to ``true``
+ */
+'files_no_background_scan' => false,
 ];