[css-color-5] Add legacy device-cmyk from older gcpm draft

css-color-5/Overview.bs

@@ -2331,6 +2331,9 @@ or any other color or monochrome output device which has been characterized.
 		It's interpreted identically to the fourth argument of the ''rgb()'' function.
 		If omitted, it defaults to ''100%''.
 
+		For <a href="https://www.w3.org/TR/2011/WD-css3-gcpm-20111129/#cmyk-colors">historical reasons</a>,
+		''device-cmyk()'' also support a <a>legacy color syntax</a>.
+
 		Typically, print-based applications will actually store the used colors as CMYK,
 		and send them to the printer in that form.
 		However, such colors do not have a colorimetric interpretation,