docs(fgs): add note for app_agency parameter of the function resource

docs/resources/fgs_function.md

@@ -264,6 +264,10 @@ The following arguments are supported:
 * `app_agency` - (Optional, String) Specifies the execution agency enables you to obtain a token or an AK/SK for
   accessing other cloud services.
 
+  -> After using this parameter, the function execution agency (`app_agency`) and the function configuration
+     agency (`agency`) can be independently set, which can reduce unnecessary performance loss. Otherwise, the same
+     agency is used for both function execution and function configuration.
+
 * `description` - (Optional, String) Specifies the description of the function.
 
 * `initializer_handler` - (Optional, String) Specifies the initializer of the function.