Add: description for compress optimization levels 6 to 9 (#2687)

# Description

Closes #https://github.com/Stirling-Tools/Stirling-PDF/issues/2672

## Checklist

- [x] I have read the [Contribution
Guidelines](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/CONTRIBUTING.md)
- [x] I have performed a self-review of my own code
- [ ] I have attached images of the change if it is UI based
- [ ] I have commented my code, particularly in hard-to-understand areas
- [ ] If my code has heavily changed functionality I have updated
relevant docs on [Stirling-PDFs doc
repo](https://github.com/Stirling-Tools/Stirling-Tools.github.io/blob/main/docs/)
- [x] My changes generate no new warnings
- [ ] I have read the section [Add New Translation
Tags](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/HowToAddNewLanguage.md#add-new-translation-tags)
(for new translation tags only)

src/main/resources/messages_zh_CN.properties

@@ -933,7 +933,8 @@ fileToPDF.submit=转换为 PDF
 compress.title=压缩
 compress.header=压缩 PDF
 compress.credit=此服务使用qpdf进行 PDF 压缩/优化。
-compress.selectText.1=手动模式 - 从 1 到 4
+compress.selectText.1=手动模式 - 从 1 到 5
+compress.selectText.1.1=In optimization levels 6 to 9, in addition to general PDF compression, image resolution is scaled down to further reduce file size. Higher levels result in stronger image compression (up to 50% of the original size), achieving greater size reduction but with potential quality loss in images.
 compress.selectText.2=优化级别：
 compress.selectText.3=4（文本图像很糟糕）
 compress.selectText.4=自动模式 - 自动调整质量以获得精确大小的PDF