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).

1 files changed, 10 insertions, 2 deletions

diff --git a/docs/api.md b/docs/api.md
index 302df5d..078594b 100644
--- a/docs/api.md
+++ b/docs/api.md
@@ -287,8 +287,16 @@ same paths: a message carries the signatures of the changes it carries, whatever
 peer was sent before, so changes sent again (a message lost on the way) arrive
 vouched for as the first time; and a node checks one it receives once it holds the
 change (a signature costs about 30 µs to check, and one vouches for its node's changes
-before it). A message carrying no changes carries no signatures. A node that stopped with changes it never sent, nor
-signed, signs them as it starts, before anything leaves.
+before it). A message carrying no changes carries no signatures; one carrying more
+than 4096 signed changes (a whole document pulled), and a delta, carry each node run's
+latest instead. A node signs the latest change of each of its runs that nothing vouches
+for as it starts, before anything leaves, and whenever its document is swapped (a drop,
+a switch, a rejoin): changes it kept unsigned, never sent.
+
+Two cases still arrive unvouched, and wait under a policy requiring signatures: a change
+vouched for only by a later change of its node that the rules refused (nodes take it,
+then can't pass the proof on, [`future-work.md`](future-work.md)); and, in one transfer,
+the changes of an epoch with more than 4096 node runs writing.
 The signatures a node has are kept in its document file (`--document-file`) with the
 document.