[css-images] Resolved w3c#461

Author: LeaVerou

css-images/Overview.bs

@@ -131,9 +131,11 @@ Resolution Negotiation: the ''image-set()'' notation {#image-set-notation}
 
 	<pre class='prod'>
 	<dfn caniuse="css-image-set">image-set()</dfn> = image-set( <<image-set-option>># )
-	<dfn>&lt;image-set-option></dfn> = [ <<image>> | <<string>> ] <<resolution>>
+	<dfn>&lt;image-set-option></dfn> = [ <<image>> | <<string>> ] [ <<resolution>> | <<x-resolution>> ]
 	</pre>
 
+	<dfn><<x-resolution>></dfn> is a <<dimension-token>> with a unit of <css>x</css>, and is treated the same as if the unit were ''dppx''.
+
 	Issue: We should add "w" and "h" dimensions as a possibility, and a "format()" function,
 	to match the functionality of HTML's <a element>picture</a>.