shutdown.8: Add a note about needed priviledges to run the command

In order to use the shutdown command, the user must
have root privileges or be a member of the operator group.

PR: 266525
Reported by: Zsolt Udvari <uzsolt at uzsolt hu>
Reviewed by: pauamma
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D36688