Update snat_rules_apply_to_nested_networks with ovn details

Related-Bug: #2051935
Change-Id: I3f1d5083429308b18cc2b4a04ffa938de20f528d
diff --git a/neutron_tempest_plugin/config.py b/neutron_tempest_plugin/config.py
index d259201..55d9d9e 100644
--- a/neutron_tempest_plugin/config.py
+++ b/neutron_tempest_plugin/config.py
@@ -81,7 +81,8 @@
                 default=False,
                 help='Whether SNAT rules apply recursively to all connected '
                 'networks. This is the default behavior for ovs and '
-                'linuxbridge drivers.'),
+                'linuxbridge drivers. OVN requires '
+                'ovn_router_indirect_snat=True setting to implement it.'),
 
     # Multicast tests settings
     cfg.StrOpt('multicast_group_range',