man: newuid and newgid: point out that root must be allocated subuids

Users may otherwise be confused and think that because the kernel
does not restrict uid mappings to the root user (within his
current uid mappings), newuidmap will ignore /etc/subuid for the
root user.  It will not.

Reported-by: Philippe Grégoire <gregoirep@hotmail.com>
Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>