Add example of client which doesn't support 2FA

[pjrobertson]

More info, see nextcloud/twofactor_gateway#249 (comment)

It was confusing/difficult for me to set up my CalDav client on macOS, as I was completely unaware of app passwords. This is an attempt to improve documentation to help other people down the line.

[tflidd]

The documentation already gives you the answer to the problem. Except for the browsers, I think only the NC clients directly support the login-flow with 2FA. It would therefore easier to supply a list of software that does support 2FA.
Not sure if we need clarification, but if it should be more general and not just one very specific software on one very specific platform.

[ChristophWurst]

👎 I agree with @tflidd. This is not about specific clients that don't support 2FA. Every client that tries to connect with the login password when 2FA is enabled will fail. It's a general problem that users have to take into consideration when they set up their client applications.

[pjrobertson]

I agree with the thoughts here in principle: that we should list the clients that do support 2FA.

However the purpose of my commits wasn't to state which clients do/don't work, but to give people who were in my situation a solution. I was unaware that after setting up 2FA I wouldn't be able to use my other apps (although in hindsight it is obvious) so I hope we can change the documentation to add a reminder that not all clients accept 2FA. I guess my issue was more with the twofactor_gateway not providing information in this case, so this PR can be closed and the solution is to add a reminder about 'Using client applications with two-factor authentication' (which I have already created a PR for)

[ChristophWurst]

Got it. Thanks! They idea was to one day only have app passwords for clients. That would of course take a lot of confusion from users. Unfortunately the login flow used by official clients is a non-standard one, hence 3rd party apps fail to connect with a little helpful authentication error.