[css-position-4][editorial] Use the newer term, it subsumes the surrounding text.

Author: tabatkins

css-position-4/Overview.bs

@@ -239,9 +239,7 @@ can be used to create a backdrop
 that hides the underlying document for an element in a top layer
 (such as an element that is displayed fullscreen).
 
-The ''::backdrop'' pseudo-element is a [=tree-abiding pseudo-element=].
-It accepts all properties.
-It inherits from its [=originating element=].
+The ''::backdrop'' pseudo-element is a [=fully styleable pseudo-element=].
 
 User agents should contain the following rules in a UA-level style sheet: