Document restore a pg 9.2 backup on a fresh GitLab 9.0 install

In GitLab 9.0 we will be defaulting the database to 9.6, but still supporting 9.2 for users with skip-auto-migrations set.

Because of that we should document how those users might restore their 9.2 db backups if they setup a new instance.

Something like:

sudo gitlab-ctl revert-pg-upgrade
sudo rm -rf /var/opt/gitlab/postgresql/data/*
sudo gitlab-ctl reconfigure
sudo gitlab-rake gitlab:backup:restore BACKUP=timestamp