[css-overflow-4] Add terms for scrollable and non-scrollable values of 'overflow'.

Author: fantasai

css-overflow-3/Overview.bs

@@ -527,7 +527,12 @@ Managing Overflow: the 'overflow-x', 'overflow-y', and 'overflow' properties</h3
 			that is visible on the screen (such as a scroll bar or a panner),
 			that mechanism will only be displayed
 			if there is overflow.
-		</dl>
+	</dl>
+
+	The ''overflow/scroll'', ''overflow/auto'', and ''overflow/hidden'' values
+	are known as the <dfn export for=overflow>scrollable values</dfn> of 'overflow'.
+	The  ''overflow/visible'' and ''overflow/clip'' values
+	are known as the <dfn export for=overflow>non-scrollable values</dfn>.
 
 	The  ''visible''/''overflow/clip'' values of 'overflow'
 	compute to ''overflow/auto''/''hidden'' (respectively)