docs: federation: all users are those there when a remake is proposedfederation

A remake that names all users names those that count in its
proposal's history, and still do at its base: the list is fixed with
the proposal, so those who vote see it, and a user brought in while
the votes run isn't on it. A fork's voters go the same way.

And what reading the design from end to end found:

* a remake does two things, not one: it throws history away and names
  who votes next;
* nothing plain is written but a node's member entry and its records
  of proposals, said where namespaces are, too;
* a node doesn't sync under a policy it doesn't know, whatever its
  version says;
* two sides that only resolve differently show by their stamps;
* on the wire a federated swarm's handshake says which epochs a node
  keeps, and a version speaks the one before its own: "no message
  changes" holds for a swarm that isn't federated;
* three more things assumed: what was seen for long kept as the node
  goes, the cost of a digest with every stamp, two wire versions
  spoken at once.

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