[css-anchor-position-1] Make anchor-name style contained w3c#9100 (w3c#9863)

Co-authored-by: Rune Lillesveen <futhark@chromium.org>

Author: lilles

css-anchor-position-1/Overview.bs

@@ -148,6 +148,7 @@ 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 there are still multiple valid [=anchor elements=]
 with the given [=anchor name=],
 the last one is chosen.