docs(swarm): correct manager address rebinding

[dargmuesli]

The multi-node guide said that rebinding an existing manager onto the private network means docker swarm init --force-new-cluster.
That command rebinds the data plane address but leaves the control plane address untouched, because it restores raft from the existing snapshot and the manager's own address is part of that snapshot.
Restarting the Docker daemon does not move it either.

This matters because a worker does not keep the address it joined on.
It replaces its stored remote with whatever the manager advertises, and libnetwork seeds the overlay's gossip cluster from that list on port 7946.
A manager advertising a public address therefore breaks the overlay on every worker, whichever address the worker was joined with.
It fails quietly, because the dispatcher session stays up on the address the worker originally dialled, so docker node ls reports the node ready while no gossip ever reaches it.

The guide now describes the two addresses separately, states that changing the control plane address needs a genuine reinitialisation, and adds a subsection on preserving secrets across one.
Production secrets are external, so the raft store is their only home and docker secret inspect does not return values.
Registry credentials have the same problem, since swarm embeds them into the service spec at deploy time rather than storing them centrally.

Verification guidance now covers the manager's control plane address and a cross-node tasks. lookup, because neither address reading on its own proves the overlay works.