docs(telemetry): say in the Alloy env example what it cannot set

Someone filling in .env.grafanacloud-alloy has no reason to know that
deployment.environment and xrpl.network.type are not env vars: they are
literals inside the tier transform, because OTTL statements do not expand
sys.env(). Left alone they ship as prod/mainnet and every dashboard
environment and network filter reads the wrong tier. The example now says
so, and notes service_instance_id needs nothing here.

docker/telemetry/.env.grafanacloud-alloy.example

@@ -25,3 +25,11 @@ GRAFANACLOUD_OTLP_KEY=
 # --- Per-node label ---
 # host label applied to this node's scraped metrics (e.g. the node's hostname).
 XRPLD_HOST_LABEL=
+
+# --- Not settable here ---
+# deployment.environment and xrpl.network.type are NOT env vars. They are
+# literals inside `otelcol.processor.transform "tier"` in alloy/config.alloy,
+# because OTTL statements do not expand sys.env(). Edit them there to match this
+# node, or every dashboard's environment and network filters will read prod and
+# mainnet. Dashboards filter on service_instance_id, which passes through from
+# the node's own [telemetry] config and needs nothing here.