Add a note to https://access.redhat.com/documentation/en-us/red_hat_openstack_platform/16.0/html-single/advanced_overcloud_customization/index#sect-Configuring_NFS_Storage that says something like: Red Hat recommends using a back end with a supported driver. Red Hat does not recommend using the generic NFS back end because its capabilities are limited when compared to a supported back end.
It's a bit late, as the change is live (and it is fine), but for more details about the need for NFS 4.x please see https://bugzilla.redhat.com/show_bug.cgi?id=1556957 which refers to https://bugzilla.redhat.com/show_bug.cgi?id=1547095
*** Bug 1556957 has been marked as a duplicate of this bug. ***
The needinfo request[s] on this closed bug have been removed as they have been unresolved for 1000 days