[css-overflow-3] Slight reword to make it blindingly obvious and not ignorable.

Author: tabatkins

css-overflow-3/Overview.bs

@@ -151,7 +151,7 @@ Scrollable overflow</h3>
 	<ul>
 		<li>the box’s own content and padding areas
 		<li>all line boxes directly contained by the box
-		<li>the margin boxes <em>and</em> border boxes
+		<li>the margin boxes <em>unioned with</em> the border boxes
 		    of all in-flow and floated boxes
 		    for which it is the containing block
 		    (accounting for transforms as described below).
@@ -190,7 +190,7 @@ Scrollable overflow</h3>
 			if it has ''transform-style: preserve-3d''
 			and the user-agent is not required to flatten it
 			based on the <a href="http://www.w3.org/TR/css3-transforms/#transform-style-property">requirements</a> in [[!CSS3-TRANSFORMS]].
-		
+
 		<dt><dfn>non-3d-preserving child</dfn>
 		<dd>
 			A child <var>C</var> of a box <var>P</var> is a non-3d-preserving-child