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. :smiley:

[deepin-ci-robot]

Hi @USFunction. Thanks for your PR.

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[deepin-bot[bot]]

Doc Check bot :green_circle: 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

The full list of commands accepted by this bot can be found here.

Needs approval from an approver in each of these files: - **[debian/deepin/OWNERS](https://github.com/linuxdeepin/dtkwidget/blob/master/debian/deepin/OWNERS)** Approvers can indicate their approval by writing `/approve` in a comment Approvers can cancel approval by writing `/approve cancel` in a comment

[deepin-bot[bot]]

TAG Bot

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

[deepin-bot[bot]]

TAG Bot

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