[css-values] Expand example

Author: fantasai

css-values/Overview.bs

@@ -1528,13 +1528,18 @@ Range Checking</h4>
 	to define their allowable values as a closed (not open) interval.
 
 	<div class=example>
-		These two are equivalent to 'width: 0px' since widths
-		smaller than 0px are not allowed.
+		Since widths smaller than 0px are not allowed,
+		these three declarations are equivalent:
 
 		<pre>
 		width: calc(5px - 10px);
+		width: calc(-5px);
 		width: 0px;
 		</pre>
+
+		Note however that ''width: -5px'' is not equivalent to ''width: calc(-5px)''!
+		Out-of-range values outside ''calc()'' are synactically invalid,
+		and cause the entire declaration to be dropped.
 	</div>
 
 <h4 id='calc-serialize'>