[#66658104] Disable Fog progress bar via Fog.credential=  -- DO NOT MERGE

This PR is to document a potential means to disable the progress bar when running in --quiet mode.

@annashipman and I have discussed it, and we're not convinced that there is a user need worth implementing this for just yet -- and due to the somewhat contentious means of overriding the Fog credentials hash it's probably not sensible to implement without a clear need.

Commit messages are worth a read.