Geo: Improve docs for re-attaching old primary to include multi-node archictures.

Problem

The instructions for re-attaching an old primary as a secondary are tailored to single node (1K) architectures.

It's difficult for customers with multi-node architectures to figure out what needs to be done to re-attach an old primary.

Proposal

Update the docs to include instructions for re-attaching an old multi-node primary as a secondary