[css-values] Put reference to previous versions in a note.

Author: tabatkins

css-values-4/Overview.bs

@@ -1750,14 +1750,15 @@ Type Checking</h4>
 	as specified in [[#calc-range]].)
 
 	Operators form sub-expressions, which gain types based on their arguments.
-	In previous versions of this specification,
+
+	Note: In previous versions of this specification,
 	multiplication and division were limited in what arguments they could take,
 	to avoid producing more complex intermediate results
 	(such as ''1px * 1em'', which is <<length>>²)
 	and to make division-by-zero detectable at parse time.
+	This version now relaxes those restrictions.
 
-	This version now relaxes those restrictions,
-	allowing arbitrary expressions,
+	[=Math functions=] allow arbitrary expressions,
 	so long as the expression as a whole resolves to a singular unit
 	according to the following rules: