[css-values-4] Properly specify how zeros are handled, so 0- becomes 0+.

Author: tabatkins

css-values-4/Overview.bs

@@ -3284,10 +3284,11 @@ Sign-Related Functions: ''abs()'', ''sign()''</h3>
 
 	The <dfn lt="abs()">abs(A)</dfn> function
 	contains one [=calculation=] A,
-	and returns the absolute value of A
-	(if A is non-negative, just A;
-	if A is negative, ''-1 * A'')
-	as the same [=CSSNumericValue/type=] as the input.
+	and returns the absolute value of A,
+	as the same [=CSSNumericValue/type=] as the input:
+	if A is positive, just A again;
+	if A is zero, 0⁺;
+	if A is negative, ''-1 * A''.
 
 	The <dfn lt="sign()">sign(A)</dfn> function
 	contains one [=calculation=] A,