[css-display] Clarify that outer display applies to the principal box. Fixes #1622.

Author: tabatkins

css-display-3/Overview.bs

@@ -151,7 +151,7 @@ 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 itself participates in <a>flow layout</a>.
+		which dictates how the [=principal box=] itself participates in <a>flow layout</a>.
 
 	<a>Text runs</a> have no <a>display type</a>.
 
@@ -294,7 +294,7 @@ Box Layout Modes: the 'display' property</h2>
 Outer Display Roles for Flow Layout: the ''block'', ''inline'', and ''run-in'' keywords</h3>
 
 	The <<display-outside>> keywords specify the element's <a>outer display type</a>,
-	which is essentially its role in <a>flow layout</a>.
+	which is essentially its [=principal box’s=] role in <a>flow layout</a>.
 	They are defined as follows:
 
 	<dl dfn-type="value" dfn-for="display, <display-outside>">