docs(network): stop pointing exposeHostPorts at otel.endpoint

`exposeHostPorts`'s worked example told the operator to set
`services.hyperhive.otel.endpoint` to a bridge address. That was true
before the collector tiers existed. It is now actively wrong: `enable`
contributes the hive collector's port to this list itself and derives
the agent-facing endpoint, while `endpoint` names where telemetry goes
after it leaves the swarm and is read by the swarm's collector. An
operator following the old example would point the upstream link at a
bridge address.

Replace it with an example that is actually the option's job -- a
host-local service the operator runs themselves -- and say plainly that
hyperhive's own telemetry needs nothing here, which docs/observability.md
already states.

nix/host-modules/hive-network.nix

@@ -100,9 +100,15 @@ in
         firewall, so an agent can connect to `''${bridgeIp}:P` (default
         `10.42.0.1:P`).
 
-        Use this to let agents reach a host-local service — e.g. an
-        OpenTelemetry collector for `services.hyperhive.otel.endpoint` (set
-        `endpoint = "http://''${bridgeIp}:P"`).
+        Use this to let agents reach a host-local service you run yourself —
+        a database, a scratch HTTP endpoint, anything listening on
+        `''${bridgeIp}:P`.
+
+        Not needed for hyperhive's own telemetry: `services.hyperhive.otel.enable`
+        contributes its collector's port here itself and derives the
+        agent-facing endpoint from the bridge address. `otel.endpoint` names
+        where telemetry goes *after it leaves the swarm*, and is read by the
+        swarm's collector — it is not a bridge address.
 
         **The host service must bind an address reachable from the bridge** —
         `0.0.0.0` or the bridge IP (`bridgeIp`) — not loopback-only. The