[css-overflow-3] Use range syntax

Part of w3c#4934

Author: frivoal

css-overflow-3/Overview.bs

@@ -494,7 +494,7 @@ Expanding ''overflow:clip'' Bounds: the 'overflow-clip-margin' property</h3>
 
 	<pre class=propdef>
 	Name: overflow-clip-margin
-	Value: <<length>>
+	Value: <<length [0,∞]>>
 	Initial: ''0px''
 	Inherited: no
 	Applies to: all elements
@@ -514,7 +514,7 @@ Expanding ''overflow:clip'' Bounds: the 'overflow-clip-margin' property</h3>
 	Values are defined as follows:
 
 	<dl dfn-type=value dfn-for=overflow-clip-margin>
-		: <dfn><<length>></dfn>
+		: <dfn><<length [0,∞]>></dfn>
 		::
 			If the element does not have ''overflow: clip''
 			(or another property defined to respect this property),