swarm: present tense + no-queue-coordinates wording

The queue's payload ceiling was justified by what the queue was about to
carry; it carries it now, so the comment says so.

The other two sites say "a hive with no queue configured". The swarm has
exactly one queue and a hive cannot lack it — only its coordinates, its
credential, or its ability to reach it. That wording is already used
everywhere else the absence is named; these two predate it.

The docs section on the agents' queue coordinates stopped at delivering
them and never said what the connection is for. It now names the subject
and the degrade rule, which is the part an operator reading an agent's
terminal at the swarm needs.

Refs #3805

docs/swarm/README.md

@@ -389,6 +389,20 @@ how it gets there. A hive that hasn't been given the queue's address for its
 agents sets none of the four and each agent logs that it has none; a half-set
 environment logs an error and the harness keeps serving.
 
+What an agent does with that connection is publish its terminal. Every row its
+own web UI renders also goes to `$SWARM.term.<hive>.<agent>`, one subject per
+agent, so a swarm-level terminal can follow one agent without subscribing to
+the swarm's whole traffic. The `<hive>` is the one the agent's client id names,
+which is the same string the broker builds its grant from. Publishing only: an
+agent talks about itself here and reads nothing. Rows aren't retained — a
+subscriber that wasn't listening missed them, the same as on the agent's own
+live stream.
+
+A row too large for the queue's `max_payload` would be refused outright and
+take the connection down with it, so the harness drops such a row's body before
+sending and leaves a marker in its place; the summary, level and icon still
+arrive. A row that's too large even without its body is logged and skipped.
+
 ### Swarm-wide forge webhooks
 
 At startup the controller registers two Forgejo hooks pointing at