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).

Start by running the provided rg command against docs\xml to locate the four remaining “To be added” placeholders under Android.Graphics.Pdf.Models.Selection. Update those source XML documentation entries, then rerun the scan to confirm that no placeholders remain in the namespace.

Written by the indexing model from the issue text.