[scroll-animations-1] Hook up view-timeline-inset to definition of progress.

Author: fantasai

scroll-animations-1/Overview.bs

@@ -329,7 +329,9 @@ spec: cssom-view-1; type: dfn;
 	are segments of a [=scroll progress timeline=]
 	that are scoped to the scroll positions
 	in which any part of the associated element’s [=principal box=]
-	intersects its nearest ancestor [=scrollport=].
+	intersects its nearest ancestor [=scrollport=]
+	(or more precisely, the relevant [=view progress visibility range=]
+	of that [=scrollport=]).
 	The startmost such scroll position represents 0% progress
 	and the endmost such scroll position represents 100% progress.