[css-color-5] Add alpha adjuster

Author: LeaVerou

css-color-5/Overview.bs

@@ -100,7 +100,10 @@ Mixing colors: the ''color-mix()'' function {#color-mix}
 </pre>
 
 <pre class='prod'>
-	<dfn>&lt;color-adjuster></dfn> = <<rgb-adjuster>> | <<hsl-adjuster>> | <<hwb-adjuster>> | <<xyz-adjuster>> | <<lab-adjuster>> | <<lch-adjuster>>
+	<dfn>&lt;color-adjuster></dfn> = [
+			  <<rgb-adjuster>> | <<hsl-adjuster>> | <<hwb-adjuster>>
+			| <<xyz-adjuster>> | <<lab-adjuster>> | <<lch-adjuster>>
+		] || alpha
 	<dfn>&lt;srgb-adjuster></dfn> = red || green || blue
 	<dfn>&lt;hsl-adjuster></dfn> = <<hue-adjuster>> || saturation || lightness
 	<dfn>&lt;hwb-adjuster></dfn> = <<hue-adjuster>> ||  whiteness || blackness
@@ -130,6 +133,7 @@ lightness means the LCH Lightness, which is the same as Lab lightness,
 but different to the HSL Lightness.
 
 Only the [=color-adjuster=]s defined for a given [=colorspace=] are available.
+The alpha adjuster is available regardless of color space.
 For example,
 it is not possible to use the HWB [=whiteness=] adjuster unless the colorspace is [=hwb=].