Update API documentation

doc/api/ci/builds.md

@@ -38,6 +38,15 @@ POST /ci/api/v1/builds/register
 curl --request POST "https://gitlab.example.com/ci/api/v1/builds/register" --form "token=t0k3n"
 ```
 
+**Responses:**
+
+| Status | Data |Description                                                                |
+|--------|------|---------------------------------------------------------------------------|
+| `201`  | yes  | When a build is scheduled for a runner                                    |
+| `204`  | no   | When no builds are scheduled for a runner (for GitLab Runner >= `v1.3.0`) |
+| `403`  | no   | When invalid token is used or no token is send                            |
+| `404`  | no   | When no builds are scheduled for a runner (for GitLab Runner < `v1.3.0`) **or** when the runner is set to `paused` in GitLab runner's configuration page |
+
 ### Update details of an existing build
 
 ```