[css-color-4] Fix #2151 defining sRGB and linearRGB

Author: svgeesus

css-color-4/Overview.bs

@@ -238,7 +238,8 @@ Resolving <<color>> Values</h3>
 <h2 id='numeric-srgb'>
 sRGB Colors</h2>
 
-	CSS colors in the sRGB color space are represented by a triplet of values--
+	CSS colors in the <dfn>sRGB</dfn> color space
+	are represented by a triplet of values--
 	red, green, and blue--
 	identifying a point in the sRGB color space [[!SRGB]].
 	This is an internationally-recognized, device-independent color space,
@@ -1811,7 +1812,11 @@ Converting sRGB colors to Lab colors</h3>
 
 	<!-- make these steps links to the appropriate section in a calculations appendix -->
 	<ol>
-		<li>Convert from sRGB to linear-light sRGB (undo gamma encoding)
+		<li>Convert from sRGB to <dfn>linear-light sRGB</dfn> (undo gamma encoding).
+			This has the same gamut as sRGB,
+			but is now additive,
+			so that arithmetic operations on the values
+			give the correct results.
 		<li>Convert from linear sRGB to CIE XYZ
 		<li>Convert from a D65 whitepoint (used by sRGB) to the D50 whitepoint used
 			in Lab, with the Bradford transform [[!Bradford-CAT]]