[selectors] Define that the structual pseudo-classes never apply to pseudo-elements.

--HG--
extra : rebase_source : a4039d6fd422dea0b7628857e0d712433402eeb1

Author: tabatkins

selectors/Overview.bs

@@ -2518,6 +2518,9 @@ Tree-Structural pseudo-classes</h2>
 	of children of its parent. When calculating the position of an element in
 	the list of children of its parent, the index numbering starts at 1.
 
+	The <a>structural pseudo-classes</a> only apply to elements in the DOM;
+	they must never match <a>pseudo-elements</a>.
+
 
 <h3 id="the-root-pseudo">
 '':root'' pseudo-class</h3>