[css-display-1] Add informative reference to DOM.

Author: fantasai

css-display-3/Overview.bs

@@ -80,6 +80,7 @@ Introduction</h2>
 	such as the DOM,
 	which can have comment nodes
 	and other types of things.
+	[[DOM]]
 	For the purposes of CSS,
 	all of these additional types of nodes are ignored,
 	as if they didn't exist.)