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

nix/host-modules/swarm-nats.nix

@@ -677,10 +677,10 @@ in
                 # The most a single publish may be, in bytes, before the
                 # server answers `-ERR 'Maximum Payload Violation'` and
                 # closes the connection — not a truncation, a dropped row
-                # and a reconnect. This queue is about to carry agent
-                # terminal rows published whole rather than split, so
-                # upstream's own default is sized to lose one of those
-                # rather than merely shorten it. Bounded from above by
+                # and a reconnect. This queue carries agent terminal rows
+                # published whole rather than split, so upstream's own
+                # default is sized to lose one of those rather than merely
+                # shorten it. Bounded from above by
                 # `max_pending`: nats-server refuses to start once this
                 # exceeds it, and widening that ceiling instead costs
                 # memory per connection, so this stays comfortably under it.