For the non-root user sysusers uses nologin as the default shell, not login. Correct the documentation to match the code.
An administrator might want to block a certain sysusers config file from being executed, e.g. to block the creation of a certain user. Only a relatively short description is added in the man page, since overrides should be relatively rare.
As pointed out by Miloslav Trmač it might be a good idea to make sure that usernames stay with in the utmp-defined limits.