[css-color-4] clarify rinbow

Author: svgeesus

css-color-4/Overview.bs

@@ -311,7 +311,7 @@ Representing Colors: the <<color>> type</h2>
 <h3 id="hue-syntax">The <<hue>> syntax</h3>
 
 Hue is represented as an angle of the color circle
-(the rainbow, twisted around into a circle).
+(the rainbow, twisted around into a circle, and with purple added between violet and red).
 
 <pre class='prod'>
 	<dfn>&lt;hue></dfn> = <<number>> | <<angle>>