Include information on updating mirror synchronization times

+ update_all_mirrors_worker_cron
+ update_all_remote_mirrors_worker_cron

See merge request !763

Document a new gotcha when using `prepend`

See merge request !963

[ci skip]
Signed-off-by: Rémy Coutable <remy@rymai.me>

CE upstream

Unmerged paths: (use "git add/rm ..." as appropriate to mark resolution)

```
both modified:   app/models/namespace.rb
both modified:   app/models/project.rb
both modified:   app/views/layouts/nav/_group_settings.html.haml
both modified:   app/views/projects/_merge_request_settings.html.haml
both modified:   app/views/projects/edit.html.haml
both modified:   app/views/shared/milestones/_summary.html.haml
both modified:   db/schema.rb
both modified:   doc/README.md
both modified:   doc/api/merge_requests.md
both modified:   lib/api/merge_requests.rb
both modified:   lib/gitlab/search_results.rb
both modified:   spec/lib/gitlab/import_export/all_models.yml
both modified:   spec/models/project_spec.rb
both modified:   spec/services/merge_requests/refresh_service_spec.rb
```

See merge request !960