add negative margin case to margin-box

Author: astearns

css-shapes-1/Overview.bs

@@ -760,10 +760,12 @@ Shapes from Box Values</h2>
 	enclosed by the outside margin edge.
 	The corner radii of this shape are determined
 	by the corresponding border-radius and margin values.
-	If the ratio of <code>border-radius/margin</code> is 1 or more,
+	If the ratio of <code>border-radius/margin</code> is 1 or more, 
+	or margin is negative, 
 	then the margin box corner radius is
-	<code>border-radius + margin</code>.
+	<code>max(border-radius + margin, 0)</code>.
 	If the ratio of <code>border-radius/margin</code> is less than 1,
+	and margin is positive, 
 	then the margin box corner radius is
 	<code>border-radius + (margin * (1 + (ratio-1)^3))</code>.