Update documentation

[mosterdt]

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This pull request addresses some documentation changes regarding the migration to 2.0 and beyond.
There is also some discussion around this in #2074 and #2040.

I will repeat here that while I agree that it's a great idea to only bind to the loopback interface by default, restricting the listener to localhost inside a docker container serves no benefit at all. It forces users to always define a mosquitto.conf. And even if mosquitto binds to every address within a docker container, it is still up to the user of the container to bind this port to their own network with e.g. -p 1883:1883.
Additionally, users need now also to explicitly allow anonymous users, or set up proper user authentication.

[mosterdt]

Removed the last commit about docker documentation as this was fixed in 12ff9d5

[tobymurray]

I ended up here for exactly the reasons mentioned by @mosterdt. Docker Hub currently contains the phrasing:

When running the image, the default configuration values are used. To use a custom configuration file, mount a local configuration file to /mosquitto/config/mosquitto.conf

I think this is much too passive, to the point where it misleads people. It seems like the default configuration should "work", so the default configuration yielding a roughly useless container is not a nice experience.

I think the proposed changes (or similar) would be both an opportunity to reduce on-boarding friction for Docker users as well as give insight into the intended security configuration. Judging by #2074 it seems like a number of people are confused, so some prescriptive "this is what'd you'd need to do to get the equivalent of 1.* containers" would be welcome!