[css-cascade-6] Specify how name-defining at-rules behave in @scope (w3c#8462)

Resolves w3c#6895.

Co-authored-by: Miriam Suzanne <miriam@oddbird.net>

Author: andruud

css-cascade-6/Overview.bs

@@ -251,6 +251,9 @@ Scoped Styles</h2>
 	up to any [=scoping limit=] elements. [=Scoping limit=] elements and
 	their descendants are not included in the [=scope=].
 
+	[=Pseudo-elements=] cannot be [=scoping roots=] or [=scoping limits=];
+	they are invalid both within <<scope-start>> and <<scope-end>>.
+  
 	Note: In contrast to [[CSS-SCOPING-1#shadow-dom|Shadow Encapsulation]],
 	which describes a persistent one-to-one relationship in the DOM
 	between a [=shadow host=] and its nested [=shadow tree=],
@@ -476,8 +479,11 @@ Scoping Styles: the ''@scope'' rule</h4>
 	are [=scoped selectors|scoped by=]
 	the selectors of the outer one.
 
-	[=Pseudo-elements=] cannot be [=scoping roots=] or [=scoping limits=];
-	they are invalid both within <<scope-start>> and <<scope-end>>.
+	Global, name-defining [=at-rules=]
+	such as ''@keyframes'' or ''@font-face'' or ''@layer''
+	that are defined inside ''@scope'' are valid,
+	but are not scoped or otherwise affected
+	by the enclosing ''@scope'' rule.
 
 <h4 id="scope-combinator">
 Scoped Descendant Combinator</h4>