[patch v2] manual: Update signal descriptions

[Florian Weimer]

* DJ Delorie:

> +@deftypevr Macro int SIGPWR
> +@cindex power event signal
> +Power lost or restored.  On s390x Linux systems, this signal is
> +generated when a machine check warning is issued, and is sent to the
> +process designated to receive ctrl-alt-del notifications.  Otherwise,
> +it is up to userspace applications to generate this signal and manage
> +notifications as to the type of power event that happened.
> +
> +The default action is to terminate the process.
> +@end deftypevr

Stefan, does s390x call these notifications “ctrl-alt-del
notifications”, or this there some other name for them?  Is this kernel
code path really active on current systems?

Thanks,
Florian