add note about scope of rule_ids.

drafts/push_csapi.rst

@@ -98,7 +98,10 @@ be redundant. Actions for the highest priority rule and only that rule apply
 (for example, a set_tweak action in a lower priority rule will not apply if a
 higher priority rule matches, even if that rule does not specify any tweaks).
 
-Rules also have an identifier, rule_id, which is a string.
+Rules also have an identifier, rule_id, which is a string. The rule_id may
+alphanumeric characters only. The rule_id is unique within the kind of rule and
+scope: rule_ids need not be unique between rules of the same kind on different
+devices.
 
 Push Rules: Actions:
 --------------------