[css-values-4] Per Oriol, fix the 'equivalent expression' to mod(). Fixes w3c#4718.

Author: tabatkins

css-values-4/Overview.bs

@@ -2749,8 +2749,11 @@ Stepped Value Functions: ''round()'', ''mod()'', and ''rem()''</h3>
 
 	Note: ''mod()'' and ''rem()''
 	can also be defined directly in terms of other functions:
-	''mod(A, B)'' is equivalent to ''calc(A - round(down, A, B))'',
+	''mod(A, B)'' is equivalent to ''calc(A - sign(B)*round(down, A*sign(B), B))''
+	(a hacky way to say "round(down) when B is positive, round(up) when B is negative),
 	while ''rem(A, B)'' is equivalent to ''calc(A - round(to-zero, A, B))''.
+	(These expressions don't always handle 0⁺ and 0⁻ correctly, though,
+	because 0⁻ semantics aren't commutative for addition.)
 
 <h4 id=round-infinities>
 Argument Ranges</h4>