[css-values-4] Rephrase minimum required calculation size to refer directly to <<calc-value>>, to handle both math functions and lots of parentheses better.

Author: tabatkins

css-values-4/Overview.bs

@@ -2837,9 +2837,8 @@ Syntax</h3>
 	on what their <<calc-sum>> arguments can contain.
 	Check the definitions of the individual functions for details.
 
-	UAs must support [=math function=] expressions of at least 20 terms,
-	where each <code>NUMBER</code>, <code>DIMENSION</code>, or <code>PERCENTAGE</code> is a term.
-	If a [=math function=] contains more than the supported number of terms,
+	UAs must support [=calculations=] of at least 20 <<calc-values>> terms.
+	If a [=calculation=] contains more than the supported number of terms,
 	it must be treated as if it were invalid.