Merge pull request #2729 from emqx/hostname-cannot-be-used-as-nodename

fix: only FQDN is allowed in node name

en_US/advanced/cluster.md

@@ -265,7 +265,8 @@ Deploy EMQX Broker cluster on two servers of s1.emqx.io, s2.emqx.io:
 | [email protected] or [email protected] | s2.emqx.io    | 192.168.0.20 |
 
 ::: tip Tip
-The format of node name is<Name@Host>, and Host must be an IP address or FQDN (server name. Domain name)
+The format of node name is<Name@Host>, and Host must be an IP address or FQDN(`hostname.domain-name`)
+The format of FQDN is `hostname.domain-name`, and it cannot be a single-layer `hostname`. For example, `abc` is invalid, while `abc.local` is valid.
 :::
 
 #### Configure [email protected] node

zh_CN/advanced/cluster.md

@@ -302,7 +302,8 @@ cluster.k8s.app_name = ekka
 | [email protected] 或 [email protected] | s1.emqx.io    | 192.168.0.10 |
 | [email protected] 或 [email protected] | s2.emqx.io    | 192.168.0.20 |
 
-**注意：** 节点名格式为 <Name@Host>, Host 必须是 IP 地址或 FQDN (主机名。域名)
+**注意：** 节点名格式为 <Name@Host>, Host 必须是 IP 地址或 FQDN。
+其中 FQDN 格式为：`主机名.域名`，不能是单层的 `主机名`。例如，`abc` 是不合法的，而 `abc.local` 是合法的。
 
 #### 配置 [email protected] 节点