docs: export matches and skips, it does not update

usage.md said items that match are updated. Export skips a match: a
change made at the source after the first export -- read state, a folder
move, an edited event, contact or Sieve script -- does not reach the target
on a second one. Say so until updating on re-export is built.

docs/usage.md

@@ -192,9 +192,10 @@ inbuxa-migrate export \
 
 Writes `ARCHIVE` into an account on a JMAP server, usually inbuxa. It keeps no
 state of its own: every run matches the archive against the target afresh.
-By default it only adds and updates -- items that match are updated, the
+By default it only adds: what the archive holds and the target lacks is
-rest are created, and anything already on the target that the archive does
+created, and anything already on the target is left as it is -- an item that
-not cover is left alone.
+matches is not updated yet, so a change made at the source after the first
+export does not reach the target on a second one.
 
 `--prune` also deletes what is on the target and not in the archive. It asks
 first; `--yes` answers for it, for scripts. Export speaks JMAP only.