Update singleton docs

Everyone can contribute. Help move this issue forward while earning points, leveling up and collecting rewards.

• Close this issue

We need to decide on what to do with the singleton docs.

Draft MR: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/11383

I think the best approach is to state that singletons are a tricky subject in JavaScript and leave a short description of object literals vs classes like suggested here: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/11383#note_29766213

Additionally we can provide examples like here: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/11383#note_29776202