[selectors] Add some dfns.

Author: tabatkins

selectors/Overview.bs

@@ -2275,7 +2275,7 @@ The current-element pseudo-class '':current''</h3>
 	The <dfn id='current-pseudo'>:current</dfn> pseudo-class represents the
 	element, or an ancestor of the element, that is currently being displayed.
 
-	Its alternate form '':current()'', like '':matches()'',
+	Its alternate form <dfn>:current()</dfn>, like '':matches()'',
 	takes a list of <a>compound selectors</a> as its argument: it represents the
 	'':current'' element that matches the argument or, if that does
 	not match, the innermost ancestor of the '':current'' element
@@ -2296,7 +2296,7 @@ The current-element pseudo-class '':current''</h3>
 <h3 id="the-past-pseudo">
 The past-element pseudo-class '':past''</h3>
 
-	The '':past'' pseudo-class represents any element that is
+	The <dfn id='past-pseudo'>:past</dfn> pseudo-class represents any element that is
 	defined to occur entirely prior to a '':current'' element.
 	For example, the WebVTT spec defines the '':past'' pseudo-class <a href="http://dev.w3.org/html5/webvtt/#the-past-and-future-pseudo-classes">relative to the current playback position of a media element</a>.
 	If a time-based order of elements is not defined by the document language,