[css-position-3] Improve example description

Author: fantasai

css-position-3/Overview.bs

@@ -531,7 +531,8 @@ Relative Positioning</h3>
 
 	<div class="example">
 
-		The following three rules are equivalent:
+		The following three rules are equivalent,
+		and shift the box ''1em'' to the left:
 
 		<pre highlight=css>
 		div.a8 { position: relative; direction: ltr; left: -1em; right: auto }