[scroll-animations-1] ScrollTimelines with zero range are inactive. w3c#7401

Author: fantasai

scroll-animations-1/rewrite.bs

@@ -113,6 +113,9 @@ spec: cssom-view-1; type: dfn;
 	along a particular axis.
 	The startmost scroll position represents 0% progress
 	and the endmost scroll position represents 100% progress.
+	If the 0% position and 100% position coincide
+	(i.e. the [=scroll container=] has no overflow to scroll),
+	the timeline is [=timeline inactive state/inactive=].
 
 	[=Scroll progress timelines=] can be referenced in 'animation-timeline'
 	anonymously using the ''scroll()'' [=functional notation=]