[css-content-3] Add missing <content-list> sub-productions

Author: cdoublev

css-content-3/Overview.bs

@@ -177,7 +177,7 @@ Inserting and replacing content with the 'content' property</h2>
 			Equal to:
 
 			<pre class=prod>
-			[ <<string>> | contents | <<image>> | <<counter>> | <<quote>> | <<target>> | <<leader()>> ]+
+			[ <<string>> | <<image>> | contents | <<quote>> | <<leader()>> | <<target>> | <<string()>> | <<content()>> | <<counter>> ]+
 			</pre>
 
 			Replaces the element's contents with one or more anonymous inline boxes