[css-values-5] Clarify that the per-element part of the random caching key must be obtained before inheritance.

Author: tabatkins

css-values-5/Overview.bs

@@ -2299,6 +2299,13 @@ Generating/Caching Random Values: the <<random-caching-options>> value</h3>
 	when invoked on different documents
 	(including refreshes of the same page).
 
+	This element-unique value must be obtained at [=computed value=] time,
+	<em>before</em> inheritance,
+	so that a random function which is unresolved by inheritance time
+	(due to containing, for example, a layout-sensitive percentage)
+	does not have a different behavior on children
+	from one that resolves immediately.
+
 	<div class=example>
 		For example, in the following stylesheet: