set-lifecycle-action: add description of type and action
In [1], <type> are described as "on_poweroff", "on_reboot",
"on_crash". but we accept "poweroff", "reboot" and "crash".
This patch adds documentation for them.
[1]: https://libvirt.org/formatdomain.html#elementsEvents
Signed-off-by: Chen Hanxiao <chenhanxiao@gmail.com>
Signed-off-by: Michal Privoznik <mprivozn@redhat.com>