[css-color-4] Further clarify host syntax, fix w3c#7567

Author: svgeesus

css-color-4/Overview.bs

@@ -4332,6 +4332,9 @@ Color Space for Interpolation</h3>
 
 
 	These features are collectively termed the <dfn export>host syntax</dfn>.
+	They are not used by this specification itself, 
+	only exposed so that other specifications can use them; 
+	see e.g. use in [[css-images-4#linear-gradients]].
 	The host syntax should define
 	what the default [=interpolation color space=]  should be for each case,
 	and optionally provide syntax for authors to override this default.