docs: revise background for external service doc (#1909)

* revise doc

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

* address the comment

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

---------

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

docs/en_US/extension/external/external_func.md

@@ -1,6 +1,8 @@
 # External Function
 
-External functions map existing services to eKuiper SQL functions through configuration. When running the rules that use external functions, eKuiper will convert data input and output according to the configuration, and call the corresponding service.
+## Background
+
+In some scenarios, we hope that eKuiper can create some internal SQL functions through hot reload, and map it as an external service, so that it can directly call the external service. Currently, eKuiper provides the configuration method to map an existing external service to an SQL function of eKuiper. When running a rule that uses an external function, the data input and output can be converted and the corresponding external service can be called.
 
 ## Configuration
 

docs/zh_CN/extension/external/external_func.md

@@ -1,6 +1,8 @@
 # 外部函数
 
-外部函数通过配置的方式，将已有的服务映射成 eKuiper SQL 函数。运行使用外部函数的规则时，eKuiper 将根据配置，对数据输入输出进行转换，并调用对应的服务。
+## 概览
+
+在某些场景里，我们希望 eKuiper 可以通过热更新的方式，创建内部的某个 SQL 函数，将其映射为外部的服务，使其在实际场景运行中可以直接调用外部服务。目前， eKuiper 提供了配置的方式，将外部已有的一个服务，映射为 eKuiper 的一个 SQL 函数。在运行使用外部函数的规则时，可以对数据输入输出进行转换，并调用对应的外部服务。
 
 ## 配置