Geo docs: Avoid out-of-date FDW tables

3 changes:

1. Ask people to wait before refreshing foreign tables, until we do gitlab#33252).
2. In the Multi-node / HA deployment with Geo section, don't assume a single secondary site.
3. Use reconfigure instead of running gitlab-rake geo:db:refresh_foreign_tables. This is slightly less painful for customers (less cognitive load). This change requires !3728 (merged)