[css-content-3] Define computation of content: normal on page margin boxes.

Author: fantasai

css-content-3/Overview.bs

@@ -114,7 +114,7 @@ Inserting and replacing content with the 'content' property</h2>
 	<dl dfn-for="content" dfn-type=value>
 		<dt><dfn>normal</dfn>
 		<dd>
-			For an element, this computes to ''content/contents''.
+			For an element or page margin box, this computes to ''content/contents''.
 
 			For ''::before'' and ''::after'', this computes to ''content/none''.