[css-values-5] Clarify that the random functions should give distinct values on different documents.

Author: tabatkins

css-values-5/Overview.bs

@@ -1294,6 +1294,11 @@ Generating/Caching Random Values: the <<random-caching-options>> value</h4>
 	but uniqueness should be likely enough
 	that authors cannot depend on elements having the same values across documents.)
 
+	Additionally, the random generation method <em>should</em>
+	generate distinct values for the same operation
+	when invoked on different documents
+	(including refreshes of the same page).
+
 	<div class=example>
 		For example, in the following stylesheet: