doc: update docs for dwatermarkhelper

[USFunction]

更新dwatermarkhelper的文档,添加示例代码

[deepin-ci-robot]

Hi @USFunction. Thanks for your PR. 😃

[deepin-ci-robot]

Hi @USFunction. Thanks for your PR.

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[deepin-bot]

Doc Check bot
🟢 Document Coverage Check Passed!

[xzl01]

1.提交邮箱用公司邮箱
2.请参照其他文档格式进行提交

[USFunction]

更新dwatermarkhelper的文档,添加示例代码

[deepin-ci-robot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: justforlxz, USFunction, xzl01

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• debian/deepin/OWNERS

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[deepin-bot]

TAG Bot

New tag: 5.6.33
DISTRIBUTION: unstable
Suggest: synchronizing this PR through rebase #586

[deepin-bot]

TAG Bot

New tag: 5.6.34
DISTRIBUTION: unstable
Suggest: synchronizing this PR through rebase #598