Add an example to docs of how to hide sensitive information from logs

In https://github.com/dbader/schedule/issues/30#issuecomment-518045325 it was proposed to have an option to customize logging in a way that secret args can be hidden. I propose to add an example on how to do this to [the docs](https://schedule.readthedocs.io/en/latest/logging.html) instead. This is in line with what other subjects like [advanced exception handling](https://schedule.readthedocs.io/en/latest/exception-handling.html).