[pkg-ntp-maintainers] ntp doc start at boot mention

[jidanni at jidanni.org]

/usr/share/doc/ntp/README.Debian.gz:
   Because of these conflicting demands and the inability of the
   maintainer to come up with a simple set of questions that cover most
   of them, the Debian package for ntp contains *no* support for any of
   these cases.

Which would probably mean it doesn't even start at boot, which it
does. So mention that.