Most of the changes are already implemented in dnf-system-upgrade.service
and packagekit-offline-update.service, so this update mostly changes the
documentation to match status quo.
This imports most of http://www.freedesktop.org/wiki/Software/systemd/SystemUpdates/
to turn it into a man page. Similarly for the man page about generators,
this will make it easier to keep up-to-date, keep a history of changes,
and make it more discoverable for end-users.
