Docs: Add a troubleshooting note about the kas-address needing a trailing slash

doc/user/clusters/agent/index.md

@@ -496,9 +496,17 @@ This error is shown if there are some connectivity issues between the address
 specified as `kas-address`, and your Agent pod. To fix it, make sure that you
 specified the `kas-address` correctly.
 
+```json
+{"level":"error","time":"2021-06-25T21:15:45.335Z","msg":"Reverse tunnel","mod_name":"reverse_tunnel","error":"Connect(): rpc error: code = Unavailable desc = connection error: desc= \"transport: Error while dialing failed to WebSocket dial: expected handshake response status code 101 but got 301\""}
+```
+
+This error occurs if the `kas-address` doesn't include a trailing slash. To fix it, make sure that the
+`wss` or `ws` URL ends with a training slash, such as `wss://GitLab.host.tld:443/-/kubernetes-agent/`
+or `ws://GitLab.host.tld:80/-/kubernetes-agent/`.
+
 ### Agent logs - ValidationError(Deployment.metadata)
 
-```plaintext
+```json
 {"level":"info","time":"2020-10-30T08:56:54.329Z","msg":"Synced","project_id":"root/kas-manifest001","resource_key":"apps/Deployment/kas-test001/nginx-deployment","sync_result":"error validating data: [ValidationError(Deployment.metadata): unknown field \"replicas\" in io.k8s.apimachinery.pkg.apis.meta.v1.ObjectMeta, ValidationError(Deployment.metadata): unknown field \"selector\" in io.k8s.apimachinery.pkg.apis.meta.v1.ObjectMeta, ValidationError(Deployment.metadata): unknown field \"template\" in io.k8s.apimachinery.pkg.apis.meta.v1.ObjectMeta]"}
 ```