docs: add mqtt source multi topic doc (#3835)

Signed-off-by: Song Gao <disxiaofei@163.com>

Author: Yisaer

docs/en_US/guide/sources/builtin/mqtt.md

@@ -194,3 +194,11 @@ Starting from version 1.5.0, eKuiper has modified the MQTT source broker configu
 
 - If you've been using an MQTT broker as a stream source in earlier versions and plan to upgrade to 1.5.0 or subsequent releases, ensure that the `server` configuration in the `etc/mqtt_source.yaml` file is correctly set.
 - If you've been relying on environment variables to determine the MQTT source address, an adjustment is required. For instance, if your broker address is `tcp://broker.emqx.io:1883`, then the environment variable should be changed from `MQTT_SOURCE__DEFAULT__SERVERS=[tcp://broker.emqx.io:1883]` to `MQTT_SOURCE__DEFAULT__SERVER="tcp://broker.emqx.io:1883"`
+
+## Listening to Multiple Topics
+
+When creating a stream, we can use the following method to listen to multiple topics simultaneously:
+
+```sql
+{"sql":"create stream my_stream (id bigint, name string, score float) WITH ( datasource = \"t1,t2\", FORMAT = \"json\", KEY = \"id\")"}
+```

docs/zh_CN/guide/sources/builtin/mqtt.md

@@ -187,3 +187,11 @@ REST API 为 eKuiper 提供了一种可编程的交互方式，适用于自动
 
 - 如希望通过配置文件配置，请确保 `etc/mqtt_source.yaml` 文件内的 `server` 已正确配置。
 - 如希望通过环境变量配置，例如针对 `tcp://broker.emqx.io:1883` 地址，配置命令应从 `MQTT_SOURCE__DEFAULT__SERVERS=[tcp://broker.emqx.io:1883]` 改为 `MQTT_SOURCE__DEFAULT__SERVER="tcp://broker.emqx.io:1883"`。
+
+## 监听多个 topic
+
+在创建流信息时，我们可以通过以下方式同时监听多个 topic:
+
+```sql
+{"sql":"create stream my_stream (id bigint, name string, score float) WITH ( datasource = \"t1,t2\", FORMAT = \"json\", KEY = \"id\")"}
+```