[css-values] Serialize all functions in summation

We previously only allowed for min and max in summation.
https://drafts.csswg.org/css-values-4/#math-function-serialize-a-summation

We can now also have clamp, sin, ..., hypot.

Author: ericwilligers

css-values-4/Overview.bs

@@ -3247,7 +3247,8 @@ Serialization</h3>
 			1. The number, if present
 			2. The percentage, if present
 			3. The dimensions, ordered by their units <a>ASCII case-insensitive</a> alphabetically
-			4. The ''min()'' and ''max()'' functions,
+			4. <a href="#comp-func">Comparison</a>, <a href="#trig-funcs">trigonometric</a>
+			  and <a href="#exponent-funcs">exponential</a> functions,
 				in the order they appeared in the original expression.
 
 		2. Serialize all the terms,