[css-display] Clarify that outer display type only governs how it acts in flow layout.

Author: tabatkins

css-display/Overview.bs

@@ -143,7 +143,7 @@ Box Layout Modes: the 'display' property</h2>
 		dictating how its descendant boxes are laid out.
 		(The inner display of a <a>replaced element</a> is outside the scope of CSS.)
 	* the <dfn export local-lt="outer">outer display type</dfn>,
-		which dictates how the box participates in its parent formatting context.
+		which dictates how the box itself participates in <a>flow layout</a>.
 
 	<a>Text runs</a> have no <a>display type</a>.