docs: improve descriptions

Co-authored-by: Zaiming (Stone) Shi <zmstone@gmail.com>

lib-ee/emqx_ee_bridge/i18n/emqx_ee_bridge_mongodb.conf

@@ -89,8 +89,8 @@ emqx_ee_bridge_mongodb {
 
   payload_template {
     desc {
-      en: "The template for formatting the outgoing messages.  If undefined, will send all the available context in JSON format."
-      zh: "用于格式化外发信息的模板。 如果未定义，将以JSON格式发送所有可用的上下文。"
+      en: "The template for formatting the outgoing messages.  If undefined, rule engine will use JSON format to serialize all visible inputs, such as clientid, topic, payload etc."
+      zh: "用于格式化写入 MongoDB 的消息模板。 如果未定义，规则引擎会使用 JSON 格式序列化所有的可见输入，例如 clientid, topic, payload 等。"
     }
     label: {
       en: "Payload template"