Add XML documentation for Android.Database

The `Android.Database` namespace contains **277** `To be added` placeholders in source XML documentation (excluding `docs\xml\index.xml`).

Example command:

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

## Update log

- 2026-09-10: Refreshed the placeholder scan from **271** to **277** (increased by **6**). Related documentation PRs: [#569](https://github.com/dotnet/android-api-docs/pull/569), [#644](https://github.com/dotnet/android-api-docs/pull/644).

Start in docs\xml, using the provided rg command to inspect the 277 `To be added` placeholders under Android.Database and the ns-Android.Database.xml files. Replace the placeholders with XML documentation, then rerun the scan and confirm that no matching placeholders remain outside docs\xml\index.xml.

Vom Indexierungsmodell aus dem Issue-Text verfasst.