Add XML documentation for Android.App.Backup

The `Android.App.Backup` namespace contains **100** `To be added` placeholders in source XML documentation (excluding `docs\xml\index.xml`).

Example command:

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

## Update log

- 2026-09-10: Refreshed the placeholder scan from **73** to **100** (increased by **27**). Related documentation PRs: [#644](https://github.com/dotnet/android-api-docs/pull/644).

Start with the provided rg command and inspect the matching files under docs\xml\Android.App.Backup and ns-Android.App.Backup.xml. Replace the 100 `To be added` placeholders with appropriate XML documentation, then rerun the scan to confirm no placeholders remain in the namespace.

Written by the indexing model from the issue text.