[css-anchor-position-1] Add implicit anchors to pseudo elements (w3c#9037)

Author: xiaochengh

css-anchor-position-1/Overview.bs

@@ -169,6 +169,12 @@ for a given positioned element.
 with the ''implicit'' keyword,
 rather than referring to some 'anchor-name' value.
 
+A ''::before'', ''::after'' or ''::backdrop'' [=pseudo-element=]
+has the same [=implicit anchor element=]
+as its [=originating element=].
+
+Note: Without this, these [=pseudo-elements=], which are often inaccessible
+by other specifications, cannot be positioned with [=implicit anchor elements=].
 
 Finding An Anchor {#target}
 -----------------