[css-color-5] Added alpha() RCS to color syntax w3c#10689

Author: svgeesus

css-color-5/Overview.bs

@@ -128,6 +128,7 @@ Introduction {#intro}
 	<dfn>&lt;color-function></dfn> = <<rgb()>> | <<rgba()>> |
 								<<hsl()>> | <<hsla()>> | <<hwb()>> |
 								<<lab()>> | <<lch()>> | <<oklab()>> | <<oklch()>> |
+								<<alpha()>> |
 								<<color()>>
 	</pre>