Add XML documentation for Android.Graphics.Pdf.Models.Selection

The `Android.Graphics.Pdf.Models.Selection` namespace contains **4** `To be added` placeholders in source XML documentation (excluding `docs\xml\index.xml`).

Example command:

```powershell
rg -n -i --glob 'Android.Graphics.Pdf.Models.Selection/**' --glob 'ns-Android.Graphics.Pdf.Models.Selection.xml' 'to be added' docs\xml
```

## Update log

- 2026-09-10: Refreshed the placeholder scan from **44** to **4** (reduced by **40**). Related documentation PRs: [#352](https://github.com/dotnet/android-api-docs/pull/352), [#644](https://github.com/dotnet/android-api-docs/pull/644).

首先针对 docs\xml 运行提供的 rg 命令，以定位 Android.Graphics.Pdf.Models.Selection 下剩余的四个“To be added”占位符。更新这些源 XML 文档条目，然后重新运行扫描，以确认该命名空间中不再有占位符。

由索引模型根据 Issue 内容生成。