Improve !help message
Improve !help message

Add the missing server argument to the help message for !whois, and make this
argument consistent across all command help messages.

(The change from irc.example.com to irc.example.net is simply because that is a
more common top-level-domain for IRC networks, and it reads almost as "example
net".)

matrix-org/matrix-appservice-irc,matrix-org/matrix-appservice-irc,matrix-org/matrix-appservice-irc