Refer to 'flow layout' directly, rather than just mentioning the types of boxes that flow layout can produce. Fixes #1470.

Author: tabatkins

css-display/Overview.bs

@@ -394,12 +394,11 @@ Generating Marker Boxes: the ''list-item'' keyword</h3>
 	the principal box's <a>outer display type</a> defaults to ''block''.
 
 	Note: In this level, as restricted in the grammar,
-	a list-item can only be a block box,
-	a block formatting context root box,
-	an inline box,
-	or an inline-block box.
+	list-items are limited to the <a>Flow Layout</a> display types
+	(''block''/''inline''/''run-in'' with ''flow''/''flow-root'' inner types).
 	This restriction may be relaxed in a future level of this module.
 
+
 <h3 id="layout-specific-display">
 Layout-Internal Display Types: the ''table-*'' and ''ruby-*'' keywords</h3>