[selectors-4] Link to ITS (as an example) from :dir(). Fixes w3c#2238.

Author: fantasai

selectors-4/Overview.bs

@@ -1834,10 +1834,14 @@ Linguistic Pseudo-classes</h2>
 The Directionality Pseudo-class: '':dir()''</h3>
 
 	The <dfn id='dir-pseudo'>:dir()</dfn> pseudo-class allows the author to write
-	selectors that represent an element based on its directionality as
-	determined by the document language.
+	selectors that represent an element based on its directionality
+	as determined by the <a>document language</a>.
 	For example, [[HTML5]] defines <a href="http://www.whatwg.org/html/selectors.html#selector-ltr">how to determine the directionality of an element</a>,
 	based on a combination of the <code>dir</code> attribute, the surrounding text, and other factors.
+	As another example, the <code>its:dir</code> and <code>dirRule</code> element
+	of the Internationalization Tag Set [[ITS20]]
+	are able to define the directionality of an element in [[XML]].
+
 	The '':dir()'' pseudo-class does not select based on stylistic
 	states&#8212;for example, the CSS 'direction' property does not affect
 	whether it matches.