[css-values-4] Clarify that zero terms cannot be removed. w3c#3741

See w3c#3741 (comment)

Author: fantasai

css-values-4/Overview.bs

@@ -4548,6 +4548,11 @@ Simplification</h4>
 				return the child.
 				Otherwise, return |root|.
 
+			Note: Zero-valued terms cannot be simply removed from a Sum;
+			they can only be combined with other values that have identical units.
+			(This is because the mere presence of a unit, even with a zero value,
+			can sometimes imply a change in behavior.)
+
 		9. If |root| is a Product node:
 
 			1. For each of |root|'s children