Add documentation on upgrading GitLab HA nodes
Add documentation on upgrading GitLab HA nodes

As mentioned in
https://gitlab.com/gitlab-org/gitlab-ce/issues/58121#note_145299901, it
wasn't obvious that upgrading an GitLab HA cluster required special
care. Link to the Omnibus documentation for upgrade instructions.

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/58121

stoplightio/gitlabhq,stoplightio/gitlabhq,mmkassem/gitlabhq,mmkassem/gitlabhq,iiet/iiet-git,iiet/iiet-git,stoplightio/gitlabhq,mmkassem/gitlabhq,iiet/iiet-git,iiet/iiet-git,stoplightio/gitlabhq,mmkassem/gitlabhq