Update custom file hooks documentation

The custom file hooks documentation could be improved:

  • make it very clear that the hooks must be on all rails hosts, especially the sidekiq hosts in a multi-host set up.
  • There is no gitlab-rails/fise_hooks.log if the hooks succeed.
  • There are entries in the sidekiq current log for FileHookWorker

Ticket: https://gitlab.zendesk.com/agent/tickets/618600

Edited by Mike Lockhart | GitLab