[css-overview-3] Use consistent punctuation. And valuedef intro line.

Author: fantasai

css-overflow-3/Overview.bs

@@ -321,7 +321,7 @@ Scrollable Overflow: the 'overflow-x', 'overflow-y', and 'overflow' properties</
 	that sets the specified values of both 'overflow-x' and 'overflow-y'
 	to the value specified for 'overflow'.
 
-	The values of these properties are:
+	Values have the following meanings:
 
 	<dl dfn-for="overflow, overflow-x, overflow-y" dfn-type="value">
 		<dt><dfn>visible</dfn>
@@ -377,8 +377,8 @@ Scrollable Overflow: the 'overflow-x', 'overflow-y', and 'overflow' properties</
 		<dd>
 			This value indicates that
 			the box’s content is clipped to the padding box,
-			but can be scrolled into view;
-			and therefore the box is a <a>scrollable box</a>.
+			but can be scrolled into view
+			(and therefore the box is a <a>scrollable box</a>).
 			However, if the user agent uses a scrolling mechanism
 			that is visible on the screen (such as a scroll bar or a panner),
 			that mechanism should only be displayed