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.

Vom Indexierungsmodell aus dem Issue-Text verfasst.