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/templates/misc/compress-pdf.html

@@ -26,6 +26,7 @@
               <div class="card mb-3">
                 <div class="card-body">
                   <h4 th:text="#{compress.selectText.1}"></h4>
+                  <p th:text="#{compress.selectText.1.1}"></p>
                   <label for="optimizeLevel" th:text="#{compress.selectText.2}"></label>
                   <select name="optimizeLevel" id="optimizeLevel" class="form-control">
                     <option value="1">1</option>