Document the sorting of jobs in stage dropdown
From https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/9760
> [Introduced][ce-9760] in GitLab 9.0.
[ce-9760]: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/9760
It makes jobs in stage dropdown to be sorted by severity.
Order: failed pending running manual canceled success skipped created
On Pipeline Graph we use order by name. On the job's page we use that order.