[css-values] Add clamp() to the list.

Author: tabatkins

css-values-4/Overview.bs

@@ -1878,7 +1878,7 @@ Computed Value</h4>
 
 	The computed value of a ''calc()'' function is the expression
 	with all components computed.
-	The computed value of a ''min()'' or ''max()'' function
+	The computed value of a ''min()'', ''max()'', or ''clamp()'' function
 	is the comma-separated list of expressions,
 	with each expression having all its component computed.