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.

Written by the indexing model from the issue text.