Add XML documentation for Android.App.Admin

The `Android.App.Admin` namespace contains **865** `To be added` placeholders in source XML documentation (excluding `docs\xml\index.xml`).

Example command:

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

## Update log

- 2026-09-10: Refreshed the placeholder scan from **841** to **865** (increased by **24**). Related documentation PRs: [#644](https://github.com/dotnet/android-api-docs/pull/644).

Start with the Android.App.Admin XML files under docs\xml and run the provided rg command to inspect the 865 `To be added` placeholders, excluding docs\xml\index.xml. Review the surrounding source XML documentation for each placeholder, then rerun the scan to confirm the targeted namespace no longer contains those placeholders.

Escrito por el modelo de indexación a partir del texto del issue.