[css-color-4] Clarify that HWB white or black at 100% is insufficient…

… for an achromatic color; it is the sum which matters. #9248

css-color-4/Overview.bs

@@ -2640,7 +2640,7 @@ HWB Colors: ''hwb()'' function</h2>
 	Values outside of these ranges
 	are not invalid;
 	hue angles outside the range [0,360) will be normalized to that range
-	and values of white or black greater than 100% will 
+	and values of white and black which sum to 100% or greater will 
 	produce achromatic colors as described below.
 
 	<!-- handling of negative white or black is unspecified