[css-values-4] Fill in the missing type details for abs() and sign().

Author: tabatkins

css-values-4/Overview.bs

@@ -3516,11 +3516,13 @@ Type Checking</h3>
 	[=Math functions=] themselves have [=CSSNumericValue/types=],
 	according to their contained [=calculations=]:
 
-	* The [=CSSNumericValue/type=] of a ''calc()'' expression
+	* The [=CSSNumericValue/type=] of a ''calc()'' or ''abs()'' expression
 		is the [=CSSNumericValue/type=] of its contained [=calculation=].
 	* The [=CSSNumericValue/type=] of a ''min()'', ''max()'', or ''clamp()'' expression
 		is the result of [=add two types|adding the types=]
 		of its comma-separated [=calculations=].
+	* The [=CSSNumericValue/type=] of a ''sign()'' expression
+		is «[ "number" → 1 ]».
 	* The [=CSSNumericValue/type=] of a ''sin()'', ''cos()'', or ''tan()'' expression
 		is «[ "number" → 1 ]».
 	* The [=CSSNumericValue/type=] of an ''asin()'', ''acos()'', ''atan()'', or ''atan2()'' expression