[css-pseudo-4] Restore idea that element-backed pseudos are always tree abiding.

The idea that element-backed pseudo-elements are a subset of
tree-abiding pseudo-elements was removed (inavertently, I think) in
24eda7a.  This restores it.

Fixes w3c#11284.

Author: dbaron

css-pseudo-4/Overview.bs

@@ -1610,8 +1610,9 @@ Element-backed Pseudo-Elements</h2>
   (and, in fact, often <em>are</em> real elements
   that are not otherwise selectable).
 
+  Element-backed pseudo-elements are always [=tree-abiding=].
   Unless otherwise specified,
-  they are fully styleable
+  they are [=fully styleable=]
   and inherit from their [=originating element=],
   just like standard [=tree-abiding pseudo-elements=];
   but they can be defined to inherit from another element instead.