[css-color-4] Clarified that once linearized, RGB spaces are additive. Fix w3c#9977

Author: svgeesus

css-color-4/Overview.bs

@@ -280,7 +280,7 @@ Value Definitions</h3>
 	<abbr title="the intensity per unit area, or 'how bright it is'">luminance</abbr>).
 
 	In an additive color space, calculations can be done
-	to accurately predict color mixing. Most RGB spaces
+	to <em>accurately predict</em> color mixing. Most RGB spaces
 	are not additive, because the components are
 	<em>gamma encoded</em>. Undoing this gamma encoding
 	produces linear-light values.
@@ -339,7 +339,7 @@ Value Definitions</h3>
 		</figcaption>
 	</figure>
 
-	RGB color spaces are additive,
+	Thus, once linearized, RGB color spaces are additive,
 	and their gamut is defined
 	by the chromaticities of the red, green and blue primaries,
 	plus the chromaticity of the <dfn export>white point</dfn>