[css-scroll-snap] More clearly specify handling of zero snap positions.

Author: fantasai

css-scroll-snap/Overview.bs

@@ -270,7 +270,9 @@ Scroll Snapping Rules: the 'scroll-snap-type' property {#snap-type}
 			is guaranteed to rest at a <a>snap position</a>
 			when there are no active scrolling operations.
 			That is, it must come to rest at a <a>snap position</a>
-			at the termination of a scroll, if possible.
+			at the termination of a scroll, if any exist.
+			(If none exist, then no snapping occurs.)
+
 			If the content changes such that the visual viewport
 			would no longer rest at a <a>snap position</a>
 			(e.g. content is added, moved, deleted, resized),