Add a missing link of the health check doc. (#5634)

docs/en/setup/backend/backend-health-check.md

@@ -60,3 +60,6 @@ We could opt to above query to check the readiness of GraphQL.
 OAP has implemented [gRPC Health Checking Protocol](https://github.com/grpc/grpc/blob/master/doc/health-checking.md).
 We could use [grpc-health-probe](https://github.com/grpc-ecosystem/grpc-health-probe) or any other tools to check the
 health of OAP gRPC services.
+
+## CLI tool
+Please follow the [CLI doc](https://github.com/apache/skywalking-cli#checkhealth) to get the health status score directly through the `checkhealth` command.

docs/en/setup/backend/backend-setup.md

@@ -116,6 +116,10 @@ to improve the meaning of the metrics.
 OAP backend cluster itself underlying is a distributed streaming process system. For helping the Ops team,
 we provide the telemetry for OAP backend itself. Follow [document](backend-telemetry.md) to use it.
 
+At the same time, we provide [Health Check](backend-health-check.md) to get a score for the health status.
+> 0 means healthy, more than 0 means unhealthy 
+> and less than 0 means oap doesn't startup.
+
 ## FAQs
 #### When and why do we need to set Timezone?
 SkyWalking provides downsampling time series metrics features.