docs: how signatures travel, as they do now

Review 4, appendix: api.md's Signed changes said a node passes on each node's latest
signature (true of deltas, not of messages carrying changes) and that every change
arrives vouched for. It now says what a message carries (its changes' signatures, each
run's latest past 4096 and for a delta), that a node signs each of its runs' latest
unvouched change at start and when its document is swapped (1277), and the two cases
that still arrive unvouched (a change vouched for only by a refused one, postponed;
more than 4096 runs writing in an epoch).

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