[css-color-4][editorial] Add a couple more examples where colors are interpolated

Author: svgeesus

css-color-4/Overview.bs

@@ -4618,6 +4618,8 @@ Color Space for Interpolation</h3>
 		* 'filter'
 		* 'animation'
 		* 'transition'
+		* ''color-mix()''
+		* [=relative color=] syntax
 	</div>
 
 	Mixing or otherwise combining colors