[scroll-animations-1] Clarify that timelines are scoped over the flattened tree. w3c#7047

Author: fantasai

scroll-animations-1/Overview.bs

@@ -552,6 +552,9 @@ spec: cssom-view-1; type: dfn;
 	In case of a name conflict on the same element,
 	[=scroll progress timelines=] take precedence over [=view progress timelines=].
 
+	Note: Like most operations in CSS besides [=selector=] matching,
+	timeline scoping occurs over the [=flattened element tree=].
+
 	ISSUE: Do we want to expand this scope to preceding siblings and/or distant cousins?
 
 # Avoiding cycles with layout # {#avoiding-cycles}