Add XML documentation for Java.Nio.FileNio.Attributes

The `Java.Nio.FileNio.Attributes` namespace contains **2** `To be added` placeholders in source XML documentation (excluding `docs\xml\index.xml`).

```powershell
rg -n -i --glob 'Java.Nio.FileNio.Attributes/**' --glob 'ns-Java.Nio.FileNio.Attributes.xml' 'to be added' docs\xml
```

## Update log

- 2026-09-10: Refreshed the placeholder scan from **204** to **2** (reduced by **202**). Related documentation PRs: [#579](https://github.com/dotnet/android-api-docs/pull/579), [#644](https://github.com/dotnet/android-api-docs/pull/644).

Run the provided rg command from the repository root to locate the two remaining `To be added` placeholders under `docs\xml` for `Java.Nio.FileNio.Attributes`. Read the surrounding XML documentation entries, replace both placeholders with the appropriate documentation, and rerun the scan to confirm that none remain in the namespace.

索引モデルが issue の本文から書いたものです。