Port to EE: Include documentation about STAGING_ENABLED variable for AutoDevops (!5536) · Merge requests · GitLab.org / GitLab · GitLab

What does this MR do?

Port to EE for https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/18675/commits

What are the relevant issue numbers?

https://gitlab.com/gitlab-org/gitlab-ce/issues/38549