[css-values-3] Define that nested calc()s are turned into parens.

Author: tabatkins

css-values/Overview.bs

@@ -1562,9 +1562,12 @@ Serialization</h4>
 
 	To serialize a ''calc()'' value:
 
-	1. Simplify the expression by
-		resolving all multiplications and divisions,
-		and combining compatible units.
+	1. Simplify the expression by:
+
+		* Replacing nested ''calc()'' values with parentheses containing their contents
+		* Resolving all multiplications and divisions
+		* Combining compatible units
+
 		(Which units are compatible can vary based on what stage of the value computation pipeline it's in.
 		For example, as a specified value, ''px'' and ''em'' are incompatible;
 		as a computed value, they're compatible.)