[css-values-5][editorial] Give an informative mention of the element-scoped behavior in functions.

Author: tabatkins

css-values-5/Overview.bs

@@ -2806,6 +2806,12 @@ Sharing (Or Not) Random Values: the <<random-key>> value</h3>
 			<li><dfn>element-scoped</dfn> adds an element-specific identifier
 				to the [=random cache name=],
 				so different elements will get different random values.
+
+				If the [=random function=] is being evaluated inside of a [=custom function=],
+				the element-specific identifier is for the "hypothetical element"
+				that the function's styles are evaluated on,
+				so ''element-scoped'' will give you different random values
+				on every function invocation.
 			<li><dfn>property-scoped</dfn> adds the property name the [=random function=] is being used on
 				to the [=random cache name=],
 				so different properties will get different random values.