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.