note about darktablerc inactivity time setting

content/preferences-settings/lighttable.md

@@ -47,6 +47,8 @@ enable disk backend for full preview cache
 generate thumbnails in background
 : Choose whether to automatically generate thumbnails in the background when the user is in the lighttable view and is inactive (no keyboard/mouse activity). Thumbnails will be generated up to the selected size (default never).
 
+: _N.B. You can set the period of activity required before thumbnail generation will start by altering the `inactivity_time` setting in `$HOME/.config/darktable/darktablerc` (default 5 seconds)_
+
 reset cached thumbnails
 : Force thumbnails to be regenerated by resetting the database. This may be needed if some thumbnails have been manually removed or have become corrupted (default off).