Merge branch 'documentation-for-build-trace' into 'master'

Document the API endpoint for gathering a build log

## What does this MR do?

Adds documentation for gathering a trace build log via the API. Documenting the following:
https://gitlab.com/gitlab-org/gitlab-ce/blob/de20bd5b31715f096db3fb0155c82b0eea992b6c/lib/api/builds.rb#L98-110

## Are there points in the code the reviewer needs to double check?

Please verify that a curl request using the same format as documented works

## Why was this MR needed?

No documentation currently exists

## What are the relevant issue numbers?

#18015 

## Screenshots (if relevant)

See merge request !4467

doc/api/builds.md

@@ -278,6 +278,30 @@ Response:
 
 [ce-2893]: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/2893
 
+## Get a trace file
+
+Get a trace of a specific build of a project
+
+```
+GET /projects/:id/builds/:build_id/trace
+```
+
+| Attribute  | Type    | Required | Description         |
+|------------|---------|----------|---------------------|
+| id         | integer | yes      | The ID of a project |
+| build_id   | integer | yes      | The ID of a build   |
+
+```
+curl -H "PRIVATE-TOKEN: 9koXpg98eAheJpvBs5tK" "https://gitlab.example.com/api/v3/projects/1/builds/8/trace"
+```
+
+Response:
+
+| Status    | Description                       |
+|-----------|-----------------------------------|
+| 200       | Serves the trace file             |
+| 404       | Build not found or no trace file  |
+
 ## Cancel a build
 
 Cancel a single build of a project