Merge branch 'doc-custom-githook-ui' into 'master'

Document custom Git hook error message

## What does this MR do?

Documents new functionality introduced by https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/5073

## Does this MR meet the acceptance criteria?

- [ ] [Documentation created/updated](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/development/doc_styleguide.md)
- Tests
  - [ ] Added for this feature/bug
  - [ ] All builds are passing
- [ ] Conform by the [style guides](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CONTRIBUTING.md#style-guides)
- [ ] Branch has no merge conflicts with `master` (if you do - rebase it please)
- [ ] [Squashed related commits together](https://git-scm.com/book/en/Git-Tools-Rewriting-History#Squashing-Commits)

See merge request !5088