DuckDNS: Updated documentation for IPv6, removed IPv6 address retrieval over server, and added more logging

[marvinKaster]

The DuckDNS documentation states that it is possible to retrieve the IPv6 address over a server. This is not possible as the docker container cannot communicate over IPv6. This PullRequest clarified this in the documentation and removed the corresponding line from the code. Moreover, more logging information was added to make it easier to find the error in the configuration.

See issue #3427 and previous comments in pull request #3442 by @agners