[css-scroll-snap-1] Add example from issue. w3c#4651

Author: fantasai

css-scroll-snap-1/Overview.bs

@@ -415,6 +415,9 @@ Re-snapping After Layout Changes</h4>
     then if one of them is focused or targetted,
     the [=scroll container=] must re-snap to that one
     and otherwise which one to re-snap to is UA-defined.
+    (The UA may, for example, track which element is snapped
+    as layout shifts align and de-align
+    the [=snap positions=] of other elements.)
 
     Scrolling required by a re-snap operation to a new or different box
     must behave and animate the same way as
@@ -1234,7 +1237,11 @@ Changes Since 31 January 2019 CR {#changes-20190131}
 				and their [=snap positions=] no longer coincide,
 				then if one of them is focused or targetted,
 				the [=scroll container=] must re-snap to that one
-				and otherwise which one to re-snap to is UA-defined.</ins></p>
+				and otherwise which one to re-snap to is UA-defined.
+				(The UA may, for example, track which element is snapped
+				as layout shifts align and de-align
+				the [=snap positions=] of other elements.)
+				</ins></p>
 			</blockquote>
 		<li id="change-2019-resnap-animation">
 			Require re-snapping to a new element to animate the same way