man: systemd-sysusers does not create home dir

Document that systemd-sysusers doesn’t create the specified home
directory, and point to systemd-tmpfiles as the service that can create
the home directory instead. (systemd-tmpfiles-setup.service is ordered
After=systemd-sysusers.service, so by the time systemd-tmpfiles would
create the home directory, sysusers would have created the owning user
already, and it should all work out.)