[css-color-5] Clarify origin color syntax is unrestricted

Author: svgeesus

css-color-5/Overview.bs

@@ -837,6 +837,7 @@ but they all follow a common structure:
 * Relative color syntax only applies to the [=modern color syntax=].
 	It <em>cannot</em> be used with <a>legacy color syntax</a>
 	and attempting to do so is an error.
+* However, the [=origin color=] can use either modern or legacy syntax.
 
 Except as specified for individual color functions,
 (for example, HWB Whiteness and Blackness return <<percentage>>),