Message when pushing to Geo secondary should be more descriptive

In gitlab-org/gitlab-ee!2954 the idea of "read-only" was made more general, but when pushing to secondary we lost some info in the message: https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/2954/diffs#d1c129df10cf33ea986071ad508d3a667e7f81da_23_22

Make the message more descriptive. Point to the primary git url and to the docs on how to set the push remote.