Geo Documentation: Add warning about /etc/gitlab/skip-auto-reconfigure when doing zero-downtime upgrades

Problem

During zero-downtime upgrades the user is asked to:

1. Create an empty file at /etc/gitlab/skip-auto-reconfigure. During software installation only, this will prevent the upgrade from running gitlab-ctl reconfigure and automatically running database migrations

   sudo touch /etc/gitlab/skip-auto-reconfigure

IMHO, this isn't without danger, namely, when they follow the zero-downtime upgrade procedure, they have to follow it every time. Or when they do not want that, they need to remove that file again

Multi-node or Geo

In case of a Geo or HA setup they are also asked to:

1. Ensure that gitlab_rails['auto_migrate'] = false is set in /etc/gitlab/gitlab.rb

Proposal

Add a warning or note informing the user they manually need to roll back those changes if they want back to upgrades with downtime.