Add docs for no downtime upgrades
Add docs for no downtime upgrades

Introduced in 9.1.0[1], we can do no downtime upgrades. No explicit
documentation on how to upgrade without downtime other than the
information on the release post.

[1] https://about.gitlab.com/2017/04/22/gitlab-9-1-released/#upgrade-barometer

gitlabhq/omnibus-gitlab,gitlabhq/omnibus-gitlab,gitlabhq/omnibus-gitlab,gitlabhq/omnibus-gitlab