Add XML documentation for Android.Window

The `Android.Window` namespace contains **76** `To be added` placeholders in source XML documentation (excluding `docs\xml\index.xml`).

```powershell
rg -n -i --glob 'Android.Window/**' --glob 'ns-Android.Window.xml' 'to be added' docs\xml
```

## Update log

- 2026-09-10: Refreshed the placeholder scan from **118** to **76** (reduced by **42**). Related documentation PRs: [#424](https://github.com/dotnet/android-api-docs/pull/424), [#554](https://github.com/dotnet/android-api-docs/pull/554), [#638](https://github.com/dotnet/android-api-docs/pull/638), [#644](https://github.com/dotnet/android-api-docs/pull/644).