[css-color-5] RCS color() was missing alpha channel keyword

Author: svgeesus

css-color-5/Overview.bs

@@ -1550,7 +1550,14 @@ the allowed [=channel keywords=] are:
 	after <a href="#required-conversion">conversion, if required</a> to relative CIE XYZ color space
 	adapted to the relevant white point.
 
-	The parameters have the following form:
+Within a [=relative color=] syntax ''color()'' function 
+using either <<predefined-rgb-params>> or
+<<xyz-params>>, an additional allowed [=channel keyword=] is:
+
+* <dfn value for="color()">alpha</dfn> is a <<number>> that corresponds to the 
+		[=origin color's=] alpha transparency. 1.0 is equivalent to 100%.
+
+The parameters have the following form:
 
 	* An <<ident>>  or <<dashed-ident>> denoting the color space.
 		If this is an <<ident>> it denotes one of the
@@ -1610,17 +1617,16 @@ the allowed [=channel keywords=] are:
 		are gamut mapped to the valid range at computed value time,
 		with a relative colorimetric intent.
 
-	* An optional slash-separated <<alpha-value>>.
-		This is interpreted the same way as the <<alpha-value>> in ''rgb()'',
-		and if omitted it defaults to ''100%''.
+	* An optional slash-separated <<alpha-value>>.If omitted, it defaults to ''100%''.
 
 	<wpt>
 		parsing/color-computed-relative-color.html
 	</wpt>
 
 	<div class="example" id="ex-RCS-XYZ">
 		For example, Relative Color Syntax in the CIE XYZ D65 colorspace
-		is used to generate a color which has exactly half the luminance
+		is used to generate a color which has 
+		the same chromaticity but exactly half the luminance
 		of the base color:
 
 		<pre class="lang-css">