Document JSON log formatter options

From https://gitlab.com/gitlab-org/gitlab-ce/issues/47981#note_135695306, there apparently is confusion about how to enable JSON log formats for our logs. We should make it clear you add this to /etc/gitlab/gitlab.rb:

gitlab_workhorse['log_format'] = 'json'
sidekiq['log_format'] = 'json'
gitlab_shell['log_format'] = 'json'
gitaly['logging_format'] = 'json'
registry['log_formatter'] = 'json'

A side point: Should we make Gitaly and Registry use log_format as well?