======Synopsis:======
[[on]] [<modes>]signoff [<serial#>] [-|^]<match> { <action> }

======Description:======
This hook is triggered whenever the client observes another client on
a common channel signing off from irc.  It only hooks once per user,
not once per channel.

======Parameters:======
|$0 |nickname of person signing off|
|$1- |signoff message|

======Other Notes:======
This hook is only triggered for other users.  To hook your own signoff,
use the [[EXIT]] hook.