[selectors-5]: allow :heading() to have An+B comma separated values

Author: keithamus

selectors-5/Overview.bs

@@ -170,16 +170,24 @@ Heading Structures: the heading pseudo-classes '':heading'', and '':heading()''<
 	</div>
 
 	As a functional pseudo-class,
-	<dfn id='heading-functional-pseudo' lt=':heading()'>:heading(<var>An+B</var>)</dfn>
+	<dfn id='heading-functional-pseudo' lt=':heading()'>:heading()</dfn>
 	notation represents elements that have a <a>heading level</a> among <var>An+B</var>.
+	The syntax is:
+
+	<pre class=prod>
+		::heading() = ::heading( <<An+B>># )
+	</pre>
 
 	<div class="example">
 		The following example styles headings with levels between 1 and 3 with a
 		font-weight of 900, while headings with levels 6 onward with font-weight of
-		500:
+		500, additionally heading levels 1 and 2 will be underlined, while 3 and
+		beyond will have no text-decoration:
 
 		<pre>:heading(-n+3) { font-weight: 900; }</pre>
 		<pre>:heading(n+6) { font-weight: 500; }</pre>
+		<pre>:heading(1, 2) { text-decoration: underline; }</pre>
+		<pre>:heading(n+3) { text-decoration: none; }</pre>
 	</div>
 
 	Note: The <a>heading level</a> might be different from an element's