Erm…it does show you the current status on your system but it also shows the full doc for the service/program even if it isn’t configured.
nixos-option services - returns a list of all the available services.
nixos-option services.zookeeper - returns a list of everything that can be set for zookeeper
nixos-option services.zookeeper.service provides the full doc for that specific option
Am I misunderstanding what you are wanting?