[css-display] Specify that the computed value of 'display' is scattered across multiple CSS specs. Fixes #1716.

Author: tabatkins

css-display-3/Overview.bs

@@ -154,7 +154,7 @@ Box Layout Modes: the 'display' property</h2>
 	Initial: inline
 	Applies to: all elements
 	Inherited: no
-	Computed value: as specified
+	Computed value: see prose in a variety of specs
 	Animatable: no
 	Media: all
 	</pre>