[Help Wanted] Update to GitLab Mattermost admin guide

Some users upgrading Mattermost using GitLab omnibus automation are surprised that gitlab.rb overwrites Mattermost setting, e.g. https://gitlab.com/gitlab-org/omnibus-gitlab/issues/1568#note_15134292

The GitLab Mattermost admin guide mentions this, but it’s not prominent.

Wondering if someone from the GitLab Mattermost community could help update the guide to make things more clear–ideally someone who knows how GitLab admins think and where best to place the information so it’ll be noticed.

A merge request can be opened directly on the documentation: https://gitlab.com/gitlab-org/omnibus-gitlab/blob/master/doc//gitlab-mattermost/README.md