Merge pull request #41195 from sftim/20230223_add_detail_headless_service

Document detail about headless Services

Author: k8s-ci-robot

content/en/docs/concepts/services-networking/service.md

@@ -866,6 +866,9 @@ either:
   * For IPv4 endpoints, the DNS system creates A records.
   * For IPv6 endpoints, the DNS system creates AAAA records.
 
+When you define a headless Service without a selector, the `port` must
+match the `targetPort`.
+
 ## Discovering services
 
 For clients running inside your cluster, Kubernetes supports two primary modes of