[css-display] Make the element/box distinction better, and rephrase precisely how run-ins behave as a result.

Author: tabatkins

css-display/Overview.bs

@@ -135,7 +135,7 @@ Box Layout Modes: the 'display' property</h2>
 	Media: all
 	</pre>
 
-	The 'display' property defines box's <dfn export>display type</dfn>,
+	The 'display' property defines an element's <dfn export>display type</dfn>,
 	which consists of the two basic qualities of how an element generates boxes:
 
 	* the <dfn export local-lt="inner">inner display type</dfn>,
@@ -303,10 +303,9 @@ Outer Display Roles for Flow Layout: the ''block'', ''inline'', and ''run-in'' k
 
 		<dt><dfn>run-in</dfn>
 		<dd>
-			The element generates a <a>run-in box</a>
-			when placed in <a>flow layout</a>.
-			Run-in elements act like inlines or blocks,
-			depending on the surrounding elements.
+			The element generates an <a>run-in box</a>,
+			which is a type of <a>inline-level box</a>
+			with special behavior that attempts to merge it into a subsequent block container.
 			See [[#run-in-layout]] for details.
 	</dl>
 
@@ -744,7 +743,7 @@ Run-In Layout</h2>
 		</pre>
 	</div>
 
-	A <a>run-in box</a> behaves exactly as an <a>inline-level box</a>, except:
+	A <a>run-in box</a> behaves exactly as any other <a>inline-level box</a>, except:
 
 	<ul>
 		<li>