[selectors] Reference 'invalid selector' from 'parse a selector', incidentally cleaning it up a bit. Fixes #3251.

Author: tabatkins

selectors-4/Overview.bs

@@ -3469,12 +3469,11 @@ Parse A Selector</h3>
 		<li>
 			Let <var>selector</var> be the result of parsing <var>source</var> as a <<selector-list>>.
 			If it does not match the grammar,
+			it's an [=invalid selector=];
 			return failure.
 
 		<li>
-			Otherwise,
-			if any simple selectors in <var>selector</var> are not recognized by the user agent,
-			or <var>selector</var> is otherwise invalid in some way
+			If |selector| is an [=invalid selector=] for any other reason
 			(such as, for example, containing an undeclared namespace prefix),
 			return failure.