The description for the samba-tool's subcommand "domain provision" is as follows: Promote an existing domain member or NT4 PDC to an AD DC. This description seems to be for the subcommand "domain dcpromo", and maybe right description is "Provision a domain." according to the domain.py script.
Closing this bug report, it is not correct, 'samba-tool domain provision --help' has always returned 'Provision a domain'.