[selectors-4] Disallow nesting :has() inside :has(). w3c#6952 (w3c#7344)

Author: byung-woo

selectors-4/Overview.bs

@@ -1194,6 +1194,8 @@ The Relational Pseudo-class: '':has()''</h3>
 	would match at least one element
 	when [=anchor element|anchored against=] this element.
 
+	'':has()'' pseudo-class cannot be nested; '':has()'' is not valid within '':has()''.
+
 	<div class='example'>
 		For example, the following selector matches only
 		<code>&lt;a></code> elements that contain an <code>&lt;img></code> child: