[css-color-5] Clarify that RCS origin colors include optional alpha

Author: svgeesus

css-color-5/Overview.bs

@@ -810,7 +810,9 @@ extends [=modern color syntax=]
 to allow existing colors to be modified
 using the color functions:
 if an <dfn>origin color</dfn> is specified,
-then each color channel can <em>either</em> be directly specified,
+then each color channel 
+(and the alpha channel, if specified) 
+can <em>either</em> be directly specified,
 or taken from the origin color
 (and possibly modified with [=math functions=]).
 
@@ -825,6 +827,7 @@ The precise details of each function's changes to accomodate [=relative colors=]
 but they all follow a common structure:
 
 * An [=origin color=] can be specified with a ''from <<color>>'' value at the start of the function.
+	This includes the optional alpha channel, if specified.
 * The origin color and the relative color need not use the same color function.
 * If an [=origin color=] is specified,
 	the remaining arguments can either be specified directly, as normal,