Refactor commits API documentation

Blocked by https://gitlab.com/gitlab-org/gitlab-ce/issues/10960 which needs to be fixed or documented.

Closes #4217 (closed)

Added ~13884 label

mentioned in issue #10421 (moved)

Added 41 commits:

• c870bb79...2becc6fa - 39 commits from branch master
• 1ede975c - Styleguide the commit status API
• b4d61ac4 - Merge branch 'master' into doc_refactor_commits_api

Added 216 commits:

• b4d61ac4...8be4e545 - 214 commits from branch master
• 831eb946 - Merge branch 'master' into doc_refactor_commits_api
• 373b277f - Add note about commit status API deprecation

mentioned in commit 3e65fa54

Status changed to merged