Add when the API changes got introduced

[ci skip]

doc/api/builds.md

@@ -257,6 +257,8 @@ Example of response
 
 ## Get build artifacts
 
+> [Introduced][ce-2893] in GitLab 8.5
+
 Get build artifacts of a project
 
 ```
@@ -279,6 +281,8 @@ Response:
 | 200       | Serves the artifacts file       |
 | 404       | Build not found or no artifacts |
 
+[ce-2893]: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/2893
+
 ## Cancel a build
 
 Cancel a single build of a project

doc/api/runners.md

 # Runners API
 
+> [Introduced][ce-2640] in GitLab 8.5
+
+[ce-2640]: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/2640
+
 ## List owned runners
 
 Get a list of specific runners available to the user.