[css-shapes] Dfn the <shape-box> values properly.

Author: tabatkins

css-shapes/Overview.bs

@@ -707,7 +707,7 @@ Shapes from Box Values</h2>
 
 	The definitions of the values are:
 
-	The <dfn>margin-box</dfn> value defines the shape
+	The <dfn value for="<shape-box>, shape-outside">margin-box</dfn> value defines the shape
 	enclosed by the outside margin edge.
 	The corner radii of this shape are determined
 	by the corresponding border-radius and margin values.
@@ -718,19 +718,19 @@ Shapes from Box Values</h2>
 	then the margin box corner radius is
 	<code>border-radius + (margin * (1 + (ratio-1)^3))</code>.
 
-	The <dfn>border-box</dfn> value defines the shape
+	The <dfn value for="<shape-box>, shape-outside">border-box</dfn> value defines the shape
 	enclosed by the outside border edge.
 	This shape follows all
 	of the normal border radius shaping rules
 	for the outside of the border.
 
-	The <dfn>padding-box</dfn> value defines the shape
+	The <dfn value for="<shape-box>, shape-outside">padding-box</dfn> value defines the shape
 	enclosed by the outside padding edge.
 	This shape follows all
 	of the normal border radius shaping rules
 	for the inside of the border.
 
-	The <dfn>content-box</dfn> value defines the shape
+	The <dfn value for="<shape-box>, shape-outside">content-box</dfn> value defines the shape
 	enclosed by the outside content edge.
 	Each corner radius of this box
 	is the larger of 0