[css-box-3] dfn and export the term 'edge'.

Author: fantasai

css-box-3/Overview.bs

@@ -123,10 +123,10 @@ The CSS Box Model {#box-model}
 
 	The perimeter of each of the four areas
 	(content, padding, border, and margin)
-	is called an “edge”,
-	and each edge can be broken down into
+	is called an <dfn export lt="box edge" local-lt="edge">edge</dfn>,
+	and each [=edge=] can be broken down into
 	a top, right, bottom, and left side.
-	Thus each <a>box</a> has four edges
+	Thus each <a>box</a> has four [=edges=]
 	each composed of four sides:
 
 	<dl export>