Add missing documentation about failure reason

changelogs/unreleased/docs-for-failure-reason-tooltip.yml

+---
+title: Add documentation for Pipelines failure reasons
+merge_request: 18352
+author:
+type: other

doc/ci/pipelines.md

@@ -73,6 +73,21 @@ cancel the job, retry it, or erase the job trace.
 
 ![Pipelines example](img/pipelines.png)
 
+## Seeing the failure reason for jobs
+
+> [Introduced][ce-5742] in GitLab 10.7.
+
+When a pipeline fails or is allowed to fail, there are several places where you
+can quickly check the reason it failed:
+
+- **In the pipeline graph** present on the pipeline detail view.
+- **In the pipeline widgets** present in the merge requests and commit pages.
+- **In the job views** present in the global and detailed views of a job.
+
+In any case, if you hover over the failed job you can see the reason it failed.
+
+![Pipeline detail](img/job_failure_reason.png)
+
 ## Pipeline graphs
 
 > [Introduced][ce-5742] in GitLab 8.11.
@@ -263,4 +278,5 @@ runners will not use regular runners, they must be tagged accordingly.
 [ce-6242]: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/6242
 [ce-7931]: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/7931
 [ce-9760]: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/9760
+[ce-17782]: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/17782
 [regexp]: https://gitlab.com/gitlab-org/gitlab-ce/blob/2f3dc314f42dbd79813e6251792853bc231e69dd/app/models/commit_status.rb#L99