[css-position-4] Whoops, left these edits in my buffer; part of the previous commit.

Author: tabatkins

css-position-4/Overview.bs

@@ -597,15 +597,18 @@ Inherited: no
 Animation Type: (see prose)
 </pre>
 
-The 'overlay' property determines
-whether an element is [=rendered in the top layer=] or not.
+When an element is [=in the top layer=],
+the 'overlay' property determines
+whether it is actually [=rendered in the top layer=] or not.
 
 <dl dfn-type=value dfn-for=overlay>
 	: <dfn>none</dfn>
 	:: The element isn't [=rendered in the top layer=].
 
 	: <dfn>auto</dfn>
-	:: The element is [=rendered in the top layer=].
+	:: The element is [=rendered in the top layer=]
+		if it is [=in the top layer=].
+
 		Rather than generating boxes as part of its normal position in the document,
 		it generates boxes as a sibling of the root element,
 		rendered "above" it.