R1:ccff47682f7c

Provide more useful guidance if a repository is clusterized into an existing multi-device cluster

Summary:
Fixes T12087. When transitioning into a clustered configuration for the first time, the documentation recommends using a one-device cluster as a transitional step.

However, installs may not do this for whatever reason, and we aren't as clear as we could be in warning about clusterizing directly into a multi-device cluster.

Roughly, when you do this, we end up believing that working copies exist on…