[css-color-4] Add exported definition of luminance

Author: svgeesus

css-color-4/Overview.bs

@@ -275,9 +275,11 @@ Value Definitions</h3>
 	An <dfn export>additive color space</dfn>
 	means that the coordinate system is linear in light intensity.
 	The <abbr title="International Commission on Illumination, = Commission internationale de l'éclairage (in French)">CIE</abbr>
-	XYZ color space is an additive color space
-	(in addition, the Y component of XYZ is the
-	<abbr title="the intensity per unit area, or 'how bright it is'">luminance</abbr>).
+	XYZ color space is an additive color space.
+	The Y component of XYZ is the
+	<dfn export>luminance</dfn>, the light intensity per unit area, 
+	or 'how bright it is'. Luminance is measured in candelas per square meter.
+	cd/m², also called <em>nits</em>.
 
 	In an additive color space, calculations can be done
 	to <em>accurately predict</em> color mixing. Most RGB spaces