[selectors-5][editorial] Add formal definition and production rule of :local-link()

Author: cdoublev

selectors-5/Overview.bs

@@ -66,9 +66,14 @@ The Local Link Pseudo-class '':local-link''</h3>
 	</div>
 
 	As a functional pseudo-class,
-	'':local-link()'' can also accept a non-negative integer as its sole argument,
-	which, if the document's URL belongs to a hierarchical scheme,
-	indicates the number of path levels to match:
+	<dfn id='local-link-functional-pseudo' lt=':local-link()'>:local-link()</dfn> can also accept a non-negative integer as its sole argument.
+
+	<pre class=prod>
+		<l>'':local-link()''</l> = :local-link( <<level>># )
+	</pre>
+
+	If the document's URL belongs to a hierarchical scheme,
+	<<level>> indicates the number of path levels to match:
 
 	<ul>
 		<li>'':local-link(0)'' represents a link element whose target is in the same origin as the document's URL