!16470 customdialogcontroller文档说明补充

Merge pull request !16470 from houhaoyu/cherry-pick-1679643733

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

@@ -36,6 +36,7 @@ CustomDialogController(value:{builder: CustomDialog, cancel?: () => void, aut
 ```ts
 dialogController : CustomDialogController = new CustomDialogController(value:{builder: CustomDialog, cancel?: () => void, autoCancel?: boolean})
 ```
+**说明**：CustomDialogController仅在作为@CustomDialog和@Component struct的成员变量，且在@Component struct内部定义时赋值才有效，具体用法可看下方示例。
 
 ### open()
 open(): void