Add note about additional networks using IPv6

When using a network as an additional network in the OCP cluster
that has an IPv6 Subnet, that subnet must be included in the router
to provide router advertisements, otherwise the interface wouldn't get
an address configured. This happens because the network manager is
configured with method=auto.

Author: MaysaMacedo

docs/user/openstack/deploy_dual_stack_cluster.md

@@ -33,6 +33,8 @@ Given the above example uses a provider network, this network can be added to th
 $ openstack router set --external-gateway dualstack <router-id>
 ```
 
+**Note**: Any additional IPv6 Subnet that is used in the OpenShift cluster, should be added to a neutron router to provide router advertisements.
+
 ## Creating Dual-Stack API and Ingress VIPs Ports for the cluster
 
 You must create the API and Ingress VIPs Ports with the following commands:

docs/user/openstack/deploy_single_stack_ipv6_cluster.md

@@ -36,6 +36,8 @@ Given the above example uses a provider network, this network can be added to th
 $ openstack router set --external-gateway v6-network <router-id>
 ```
 
+**Note**: Any additional IPv6 Subnet that is used in the OpenShift cluster, should be added to a neutron router to provide router advertisements.
+
 ## Creating IPv6 API and Ingress VIPs Ports for the cluster
 
 You must create the API and Ingress VIPs Ports with the following commands: