docs/services: the store export is unconditional, and there is no eval refusal

`5478e0bf` (`fix(#3554)`) made both store exporters unconditional and removed
the "somewhere to send" assertion, and touched no documentation — its diffstat
is four `nix/` files. `services.md` § *Telemetry collector (OTEL)* still
described the old shape in two places.

① "it writes the store above and exports to `otel.endpoint`, doing both when
both are configured" — only the upstream is conditional. `exporterNames`
(swarm-otel.nix:231) always carries the store exporter, with a comment saying
why: `deploy.victoriametrics.enable` means "this host RUNS the store", and a
swarm has one either way.

② "With neither `otel.endpoint` nor the store enabled, this module refuses the
collector at eval" — that assertion is gone. swarm-otel.nix:693 records the
removal at the head of the assertions list, and `5478e0bf`'s message states it
was a ruling rather than an oversight: a collector on a host of its own is a
supported shape, so refusing to build it would have made the fix illegal
exactly where the bug bit hardest. `git grep` finds no surviving assertion of
that shape.

The replacement paragraph states the invariant an operator can act on — the
exporters have no gate, the stores are addressed by swarm name — and keeps the
failure it prevents, since "an absent exporter is not an error" is the part that
made the old bug silent.

The stale claim dates to `80c9118f` (2026-08-18), 13 days before the behaviour
changed under it.

Closes #4214.

docs/swarm/services.md

@@ -156,10 +156,9 @@ intended writer.
 ### Telemetry collector (OTEL)
 
 The swarm's collector receives from every hive's own collector and is the
-only process that decides where telemetry goes: it writes the store above
+only process that decides where telemetry goes: it always writes the stores
-and exports to `otel.endpoint`, doing both when both are configured. It
+above, and exports to `otel.endpoint` as well when that's set. It holds
-also holds the upstream credential, which is why no hive and no agent
+the upstream credential, which is why no hive and no agent needs one.
-needs one.
 
 It runs in a `swarm-otel` container. Its `swarm.otel.port` defaults to
 `4319` rather than OTLP's usual `4318`, which the hive tier uses — swarm
@@ -178,9 +177,11 @@ gateway either way.
 | `swarm.otel.domain` | Only to rename it — the default already resolves correctly for every hive in the swarm. |
 | `swarm.otel.port`   | Only if something else on the services host already claims `4319`.                      |
 
-With neither `otel.endpoint` nor the store enabled, this module refuses
+**Both store exporters are unconditional**, and `deploy.victoriametrics.enable`
-the collector at eval — a tier that receives samples and drops them
+doesn't gate them: that option says this host _runs_ the store, while the swarm
-looks healthy while losing data.
+has one either way, reached by its swarm name through the gateway. Gating on it
+once left a collector on any other host with no exporter at all — receiving from
+every hive and dropping it, silently, because an absent exporter isn't an error.
 
 Agent-side configuration, and what a hive's own collector does, are in
 [`../scheduler/observability.md`](../scheduler/observability.md).