https://bugzilla.redhat.com/show_bug.cgi?id=873344 suggested that
the grouping 'boot', 'shutdown', 'reboot'; as well as the grouping
'start', 'stop', 'restart'; might be easier to remember than the
current mix of 'start', 'shutdown', 'reboot'.

Also, touch up the wording of 'reboot' to be more accurate.

* tools/virsh-domain.c (domManagementCmds): Add other command names.
* tools/virsh.pod (start, shutdown, reboot): Document the aliases.