docs: config/auth: clarify auth_username, etc

[half-duplex]

Description

Add an example for having Sopel authenticate to ZNC, inspired by a user in IRC confused by auth_username not working with auth_method=server.

Also corrects/clarifies/creates a few other things:

• auth_password: used for server auth too, not just user
• please click the directives, I don't want to have to remember to update things in two places
• auth_method=userserv also uses auth_target
• when is auth_username ignored/required (previously: infinite docloop)

Checklist

• I have read CONTRIBUTING.md
• I can and do license this contribution under the EFLv2
• No issues are reported by make qa (runs make quality and make test)
• I have tested the functionality of the things this change touches

[half-duplex]

For 8.0:

<+dgw> Looks like clients that offer separate username & password fields for bouncers, like Colloquy, just concatenate the two together with a : for the PASS command.

I don't think it's worth adding auth_method=bouncer or anything, but can we think of a case where if auth_method==server and auth_username: auth_password=auth_username+":"+auth_password would cause problems?

[dgw]

The only thing I don't understand about this patch is the addition of "allowed or" in auth_username's description.

[Exirel]

Happy to see more/better documentation!

[half-duplex]

dgw, I phrased it badly, but the intent was to remove the implication that "it's allowed, and sometimes it's required" since it's never optional.
Maybe we should log a warning when a username is provided with a method that ignores it?

[dgw]

I had already marked these changes as viewed, so I don't know why there wasn't also a (re-)review to go with that. Oh well, squash away.