Negative commit message push rule docs

What does this MR do?

CE version of this community contribution: https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/14004

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation created/updated or follow-up review issue created

Created by me since I forgot to hit the "merge" button some weeks ago and our policies have changed since!

lupine@gitlab-t470p:~/dev/gitlab.com/gitlab-org/gdk-ce/gitlab$ git remote remove maikel-v-ee
lupine@gitlab-t470p:~/dev/gitlab.com/gitlab-org/gdk-ce/gitlab$ git remote add maikelv-ee git@gitlab.com:maikelv/gitlab-ee.git
lupine@gitlab-t470p:~/dev/gitlab.com/gitlab-org/gdk-ce/gitlab$ git fetch maikelv-ee patch-25
remote: Enumerating objects: 441555, done.
remote: Counting objects: 100% (441555/441555), done.
remote: Compressing objects: 100% (109497/109497), done.
remote: Total 441555 (delta 344060), reused 420137 (delta 326053)
Receiving objects: 100% (441555/441555), 117.12 MiB | 3.78 MiB/s, done.
Resolving deltas: 100% (344060/344060), completed with 10191 local objects.
From gitlab.com:maikelv/gitlab-ee
 * branch                    patch-25   -> FETCH_HEAD
 * [new branch]              patch-25   -> maikelv-ee/patch-25
lupine@gitlab-t470p:~/dev/gitlab.com/gitlab-org/gdk-ce/gitlab$ git checkout -b negative-commit-message-push-rule-docs
Switched to a new branch 'negative-commit-message-push-rule-docs'
lupine@gitlab-t470p:~/dev/gitlab.com/gitlab-org/gdk-ce/gitlab$ git cherry-pick 6828e20895de15e382cc0829652ca91f1dbd7eb9
[negative-commit-message-push-rule-docs 4d8f28f6e9b] Document the negative commit message push rule for the API.
 Author: Maikel <maikel@mediamoose.nl>
 Date: Thu Jun 6 11:12:12 2019 +0000
 1 file changed, 27 insertions(+), 24 deletions(-)
lupine@gitlab-t470p:~/dev/gitlab.com/gitlab-org/gdk-ce/gitlab$ git cherry-pick c8a2dc2495790fc0a05ea7d0ad212e9e0dfeb4af
[negative-commit-message-push-rule-docs b6f7ad238a1] Add the changelog entry.
 Author: Maikel <maikel@mediamoose.nl>
 Date: Thu Jun 6 11:28:39 2019 +0000
 1 file changed, 5 insertions(+)
 create mode 100644 changelogs/unreleased/api-doc-negative-commit-message-push-rule.yml
lupine@gitlab-t470p:~/dev/gitlab.com/gitlab-org/gdk-ce/gitlab$ git status
On branch negative-commit-message-push-rule-docs
nothing to commit, working tree clean