[selectors-4] Clarify that selector matching happens as strings.

Author: fantasai

selectors/Overview.bs

@@ -580,7 +580,8 @@ Data Model</h3>
 	this may be referred to as the "document tree" or "source document".
 
 	Each element may have any of the following five aspects,
-	which can be selected against:
+	which can be selected against,
+	all of which are matched as strings:
 
 	<ul>
 		<li>The element's type (also known as its tag name).