[css-anchor-position] Layout containment also scopes anchor names. w3c#9349

Author: tabatkins

css-anchor-position-1/Overview.bs

@@ -150,7 +150,8 @@ Not all elements are capable of being [=anchor elements=]
 for a given positioned element,
 so a name can be reused in multiple places
 if the usages are scoped appropriately.
-[=Anchor names=] are style-contained, [=scoped to a sub-tree=].
+If the element has either [=style containment=] or [=layout containment=],
+the 'anchor-name' property is [=scoped property|scoped=] [=scoped to a subtree|to a subtree=].
 If there are still multiple valid [=anchor elements=]
 with the given [=anchor name=],
 the last one is chosen.