[css-color-4] Clarify which component in XYZ corresponds to brightness

css-color-4/Overview.bs

@@ -4298,7 +4298,7 @@ The Predefined CIE XYZ Color Spaces: the ''xyz-d50'', ''xyz-d65'', and ''xyz'' k
  the reference white for ''xyz-d65'' and ''xyz'' is [=D65=].
 
  Values greater than 1.0/100% are allowed and must not be clamped;
- they represent colors brighter than diffuse white.
+ colors where Y is greater than 1.0 represent colors brighter than diffuse white.
  Values less than 0/0% are uncommon,
  but can occur as a result of chromatic adaptation,
  and likewise must not be clamped.