Update alert summary

Co-authored-by: Bartlomiej Plotka <bwplotka@gmail.com>

examples/alerts/alerts.yaml

@@ -316,7 +316,7 @@ groups:
       description: Thanos Sidecar {{$labels.instance}} is unhealthy for more than
         {{$value}} seconds.
       runbook_url: https://github.com/thanos-io/thanos/tree/main/mixin/runbook.md#alert-name-thanossidecarunhealthy
-      summary: Thanos Sidecar is unhealthy.
+      summary: Thanos Sidecar cannot access Prometheus, even though Prometheus seems healthy and has reloaded WAL.
     expr: |
       time() - max by (pod, job, instance) (thanos_sidecar_last_heartbeat_success_time_seconds{job=~".*thanos-sidecar.*"}) >= 240
       AND on (pod) (