usrloc: Clarify docs for MI 'ul_cluster_sync'

(cherry picked from commit 8f32842)

modules/usrloc/doc/usrloc_admin.xml

@@ -1656,7 +1656,8 @@ modparam("usrloc", "contact_refresh_timer", true)
 		<function moreinfo="none">ul_cluster_sync</function>
 		</title>
 		<para>
-		This command will only take effect if the module is running under a
+		This command will only take effect if the target OpenSIPS instance is
+		paired with a hot backup instance, while running under a
 		cluster-enabled <xref linkend="param_working_mode_preset"/>.
 		</para>
 		<para>