[css-values-4] Clarify that <position> computation is also deferred to css-backgrounds currently.

Author: fantasai

css-values-4/Overview.bs

@@ -2824,7 +2824,7 @@ Combination of <<image>></h4>
 
 	The <dfn><<position>></dfn> value specifies the position of a object area (e.g. background image)
 	inside a positioning area (e.g. background positioning area).
-	It is interpreted as specified for 'background-position'. [[!CSS3-BACKGROUND]]
+	It is computed and interpreted as specified for 'background-position'. [[!CSS3-BACKGROUND]]
 
 	<pre class=prod>
 		<<position>> = [