[css-cascade-4] Make it even clearer that the value stages only apply to things in the flat tree.

Author: tabatkins

css-cascade-4/Overview.bs

@@ -442,7 +442,7 @@ Value Processing</h2>
 
 	Once a user agent has parsed a document and constructed a document tree,
 	it must assign,
-	to every element in the tree,
+	to every element in the [=flat tree=],
 	and correspondingly to every box in the formatting structure,
 	a value to every property that applies to the target media type.