Add XML documentation for Java.Nio.FileNio

The `Java.Nio.FileNio` namespace contains **420** `To be added` placeholders in source XML documentation (excluding `docs\xml\index.xml`).

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

## Update log

- 2026-09-10: Refreshed the placeholder scan from **416** to **420** (increased by **4**). Related documentation PRs: [#439](https://github.com/dotnet/android-api-docs/pull/439), [#459](https://github.com/dotnet/android-api-docs/pull/459), [#503](https://github.com/dotnet/android-api-docs/pull/503), [#644](https://github.com/dotnet/android-api-docs/pull/644).

Start in docs\xml and run the provided rg command for the Java.Nio.FileNio namespace. Replace the 420 reported "To be added" XML documentation placeholders with appropriate documentation, then rerun the scan to confirm none remain.

Written by the indexing model from the issue text.