Hi Kevin,
The documentation is correct. When adding an AG cluster, you do use the cluster name and not the VNN to add the environment. Once the environment is added, the Delphix Engine will list all of the AGs, that have Listeners, that are running on the SQL instances on that cluster.
So for example, if you had two SQL instances running in the cluster and both had AGs with Listeners, both SQL instances would be listed in the GUI. Or. if you had just once clustered SQL instance with two AGs, both of those would be listed.
Hope that helps!
-darin