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