clarified default behavior and how fractional scrolling looks for the…

… "enable smooth scrolling for lighttable thumbnails" setting

content/preferences-settings/lighttable.md

@@ -45,7 +45,7 @@ enable disk backend for full preview cache
 : If enabled, darktable writes full preview images to disk (`.cache/darktable/`) when evicted from the memory cache. Note that this can take a lot of storage (several gigabytes for 20k images) and darktable will never delete cached images. It's safe to delete these manually if you want. Enabling this option will greatly improve lighttable performance when zooming an image in full preview mode (default off).
 
 enable smooth scrolling for lighttable thumbnails
-: If enabled, use fractional scrolling when navigating through the thumbnails in the filemanager layout for a smoother experience. This is most useful when scrolling via a touchpad or other input device and not a mouse with a scroll wheel.
+: If enabled, use fractional scrolling when navigating through the thumbnails in the filemanager layout for a smoother experience. This means that when enabled, the top row of thumbnails may only show partial images as the scrolling is not aligned with each row. This setting is most useful when scrolling via a touchpad or other input device and not a mouse with a scroll wheel (default off).
 
 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).