man: update description of exit.target

man: update description of exit.target

exit.target is now used for both system and user sessions,
so remove "on user service manager exit". Also reword that
paragraph: services will be killed before the manager exits,
even if they do not conflict with shutdown target, but we
recommend that they conflict with shutdown target so that
systemd schedules them to be stopped immediately when starting
to exit.

In the first paragraph, containers should be mentioned last,
and the more general systems first.