chore: add help text for mustVerifyEmailOnRegistration option (#5410)

#### What type of PR is this?

/area core
/kind improvement
/milestone 2.13.0

#### What this PR does / why we need it:

为用户设置中 **注册需验证邮箱** 选项添加提示。

#### Does this PR introduce a user-facing change?

```release-note
None
```

application/src/main/resources/extensions/system-setting.yaml

@@ -98,7 +98,8 @@ spec:
           value: false
         - $formkit: checkbox
           name: mustVerifyEmailOnRegistration
-          label: "注册需验证邮箱（请确保启用邮件通知）"
+          label: "注册需验证邮箱"
+          help: "需要确保已经正确配置邮件通知器"
           value: false
         - $formkit: roleSelect
           name: defaultRole