In older revision of documentation (up to 2019-02-18), OpenVPN basic was using UCI and the configuration was located in /etc/config/openvpn.
With new realeases, it is a conf-based setup. The configuration is located in /etc/openvpn/server.conf and /etc/config/openvpn is not used anymore.
The drawback of the new setup is that you can't manage the service with LUCI via luci-app-openvpn and you can't have more than one configuration.
Is there any reason why this move was made? It is quite confusing for newcomers.
Il you want to allow multiple configurations and keep conf-based setup, there sould be an entry in /etc/config/openvpn to allow senable/disable the configuration and starting/stopping the service.
config openvpn 'basic_server'
option config '/etc/openvpn/server.conf'
At best, the configuration should be reverted to UCI allowing to edit VPN parameters from the WEB interface. I made it and it works very well.