Update comments to reflect changes in 8e0d32a4a1.

Update comments to reflect changes in 8e0d32a4a1.

Commit 8e0d32a4a1 fixed an issue by allowing the replication origin to be
created while marking the table sync state as SUBREL_STATE_DATASYNC.
Update the comment in check_old_cluster_subscription_state() to accurately
describe this corrected behavior.

Author: Amit Kapila <[email protected]>
Reviewed-by: Michael Paquier <[email protected]>
Backpatch-through: 17, where the code was introduced
Discussion: https://postgr.es/m/CAA4eK1+KaSf5nV_tWy+SDGV6MnFnKMhdt41jJjSDWm6yCyOcTw@mail.gmail.com
Discussion: https://postgr.es/m/[email protected]