RE: [PoC] pg_upgrade: allow to upgrade publisher node

Dear Shlok,

Thanks for testing the feature!

> 
> I tested a test scenario:
> I started a new publisher with 'max_replication_slots' parameter set
> to '1' and created a streaming replication with the new publisher as
> primary node.

Just to confirm what you did - you set up a physical replication and the
target of pg_upgrade was set to the primary, right?

I think we can assume that new cluster (target of pg_upgrade) is not used yet.
The documentation describes the usage [1] and it says that we must initialize
the cluster (at step 4) and then run the pg_upgrade (at step 10).

Therefore I don't think we should document anything about it.

[1]: https://www.postgresql.org/docs/devel/pgupgrade.html#:~:text=Initialize%20the%20new%20PostgreSQL%20cluster

Best Regards,
Hayato Kuroda
FUJITSU LIMITED