[scroll-animations-1] source is updated upon request and in sync with currentTime #8204 (#8541)

Author: fantasai

scroll-animations-1/Overview.bs

@@ -280,6 +280,9 @@ spec: cssom-view-1; type: dfn;
 
 	A {{ScrollTimeline}}’s {{AnimationTimeline/duration}} is 100%.
 
+	The values of {{ScrollTimeline/source}} and {{AnimationTimeline/currentTime}}
+	are both computed when either is requested or updated.
+
 ## Named Scroll Progress Timelines ## {#scroll-timelines-named}
 
 	[=Scroll progress timelines=] can also be defined on the [=scroll container=] itself,
@@ -570,6 +573,9 @@ spec: cssom-view-1; type: dfn;
 
 	ISSUE: Figure out how to incorporate fit/inset abilities.
 
+	The values of {{ViewTimeline/subject}}, {{ScrollTimeline/source}}, and {{AnimationTimeline/currentTime}}
+	are all computed when any of them is requested or updated.
+
 ## Named View Progress Timelines ## {#view-timelines-named}
 
 	[=View progress timelines=] can also be defined declaratively