Add XML documentation for Android.Nfc

The `Android.Nfc` namespace contains **147** `To be added` placeholders in source XML documentation (excluding `docs\xml\index.xml`).

Example command:

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

## Update log

- 2026-09-10: Refreshed the placeholder scan from **144** to **147** (increased by **3**). Related documentation PRs: [#602](https://github.com/dotnet/android-api-docs/pull/602), [#644](https://github.com/dotnet/android-api-docs/pull/644).

Run the provided rg command first to enumerate the 147 placeholders under docs\xml for Android.Nfc, excluding docs\xml\index.xml. Review the matching Android.Nfc/** and ns-Android.Nfc.xml files and the related documentation PRs #602 and #644 for conventions. Done means the Android.Nfc placeholder count is addressed and the scan confirms the intended entries no longer say “To be added”.

Vom Indexierungsmodell aus dem Issue-Text verfasst.