Add comment about geo_primary_role for older installations

See the general Documentation guidelines http://docs.gitlab.com/ce/development/doc_styleguide.html

What does this MR do?

Add a small comment to documentation of geo_primary_role to help customers who are trying to install on an older version of GitLab.

We had a customer run into this here https://gitlab.zendesk.com/agent/tickets/74380

/cc @brodock