Exception logging refactoring: documentation updates and log file tracking

After we merged Improved exception logging !17819 (merged), a several additional steps should be performed.

It is basically what is described in !17819 (merged):

1. Consider log retention settings. By default, Omnibus will rotate any logs in /var/log/gitlab/gitlab-rails/*.log every hour and keep at most 30 compressed files. On GitLab.com, that setting is only 6 compressed files. These settings should suffice for most users, but you may need to tweak them in Omnibus GitLab.
2. If you add a new file, submit an issue to the production tracker or a merge request to the gitlab_fluentd project. See this example.
3. Be sure to update the GitLab CE/EE documentation and the GitLab.com runbooks.