[css-color-5] Explicitly clarify (rather than inferring) that there is no RCS for device-cmyk() w3c#9555

Author: svgeesus

css-color-5/Overview.bs

@@ -1240,6 +1240,8 @@ Relative sRGB color syntax is <em>only</em> applicable to the non-legacy RGB syn
 
 </div>
 
+There is no relative ''device-cmyk()'' syntax.
+
 <h3 id="relative-HSL">Relative HSL Colors</h3>
 
 The grammar of the [=modern color syntax=] ''hsl()'' and ''hsla()'' functions is extended as follows: