[css-scroll-snap-1] Clarify overlarge snapping example. w3c#6863

Author: fantasai

css-scroll-snap-1/Overview.bs

@@ -850,6 +850,14 @@ Snapping Boxes that Overflow the Scrollport</h4>
                 Because the first and last snap positions are part of ranges taller than the viewport,
                 the viewer is allowed to scroll freely
                 between the top and bottom of each range.
+
+                Any position in that range is a valid [=snap position=],
+                which can be snapped to when it is the nearest position;
+                however, if the element is targetted directly
+                (such as by a fragment ID or a scrolling API),
+                the UA will land on the bolded position,
+                which corresponds to the ideal requested alignment
+                of the element’s [=snap area=] within the [=snapport=].
             </figcaption>
         </figure>