Document RFC2812-style service commands

[progval]

After discussion in #78, it looks like RFC2812-style service commands are still use, either for actual RFC2812-style services (eg. IRCnet), or for Anope/Atheme.

(I also think they are a good idea in general (avoids the /msg NickServ type of leaks on misconfigured servers), but that's not very irrelevant)

So we should document the commands here.

[emersion]

It doesn't seem like these commands are widely supported, e.g. SQUERY on OFTC gives "unknown command".