Geo: add note updating primary will cause FDW to go out-of-date
When a user performs an upgrade on the primary, it will perform some migrations (in zero-downtime upgrades and also normal upgrades).
These migrations will propagate through database replication to the secondary nodes. Those will then suddenly report their FDW schema is out-of-date.
Proposal
Add a note in the docs this might happen.
For zero-downtime updates it should not be a problem, because the migrations done should be compatible. So the user can run rake geo:refresh_foreign_tables
, but it's not needed.
For normal upgrades I'm not sure. Maybe FDW is the least of the problems, cause there might have been other breaking migrations.