docs(#3265): observability.md still said the endpoint was required

Review catch: this PR relaxed the `otel.endpoint` assertion and staled the
canonical OTEL reference in the same stroke — `docs/observability.md` is
what CLAUDE.md points readers at for "what OTEL options are available",
and it still said required-full-stop while the new swarm/services.md
section said a local store satisfies it.

Also corrects the option's own description in otel.nix, which said the
same thing and renders into the generated options doc. Grepping the
reviewer's phrasing did not find that one; grepping the claim did.

Records the second destination where the "endpoint is where telemetry
ultimately goes" paragraph makes its claim, rather than only in the new
section a reader may not reach.

nix/host-modules/otel.nix

@@ -36,8 +36,15 @@
       default = "";
       example = "https://collector.example.com/otel";
       description = ''
-        OTLP collector endpoint, set as `OTEL_EXPORTER_OTLP_ENDPOINT`
-        for every agent. Required when `enable` is true.
+        Upstream OTLP endpoint, set as `OTEL_EXPORTER_OTLP_ENDPOINT` for
+        every agent.
+
+        Required when `enable` is true, **unless** this host runs the
+        swarm's metrics store
+        ({option}`services.hyperhive.swarm.victoriametrics.enable`) — that
+        store is a destination in its own right, and with both configured
+        telemetry goes to both. With neither, `enable` is refused rather
+        than silently exporting nowhere.
       '';
     };