API: document GitLab version for each endpoint
Recently there were several confusions about endpoints being listed on https://docs.gitlab.com, but not yet deployed to GItLab.com:
- https://gitlab.com/gitlab-org/gitlab-ce/issues/29387
- https://gitlab.com/gitlab-org/gitlab-ce/issues/29309
- (and possibly more)
As mentioned on chat we should improve the docs and mention for each endpoint of the API, in which version it was added/changed.