Add explicit symptom and solution when 'Geo database has an outdated FDW remote schema'

After upgrading GitLab on http://selective-secondary.gogitlab.com to the latest RC, I noticed the following message at http://selective-secondary.gogitlab.com:

Geo database has an outdated FDW remote schema. It contains 229 of 236 expected tables. Please refer to Geo Troubleshooting.

We don't have anything explicitly mentioned in https://docs.gitlab.com/ee/administration/geo/replication/troubleshooting.html for how to handle this problem.

The solution is to run sudo gitlab-rake geo:db:refresh_foreign_tables