fixed afc1b1c8 from https://gitee.com/esterzhou/docs/pulls/17173

Update doc (16424)
Signed-off-by: Nester.zhou <ester.zhou@huawei.com>

en/application-dev/reference/arkui-ts/ts-methods-custom-dialog-box.md

@@ -33,6 +33,9 @@ CustomDialogController(value:{builder: CustomDialog, cancel?: () => void, aut
 ```ts
 dialogController : CustomDialogController = new CustomDialogController(value:{builder: CustomDialog, cancel?: () => void, autoCancel?: boolean})
 ```
+> **NOTE**
+> 
+> **CustomDialogController** is valid only when it is a member variable of the **@CustomDialog** and **@Component** decorated struct and is defined in the **@Component** decorated struct. For details, see the following example.
 
 ### open()
 open(): void