DR: document process for bring a demoted primary back online
We're making it easier to promote a secondary to primary in 2-node (1 primary, 1 secondary) configurations https://gitlab.com/gitlab-org/gitlab-ee/issues/1921 and for n-node (1 primary, n-1 secondary) configurations https://gitlab.com/gitlab-org/gitlab-ee/issues/4130. In most disaster situations (unplanned), the customer will want to bring the original instance back online because they would presumably have most users geographically closest to the original primary.
We should document and test the procedure for this.
Edited by James Ramsay (ex-GitLab)