Add XML documentation for Java.Nio

The `Java.Nio` namespace contains **301** `To be added` placeholders in source XML documentation (excluding `docs\xml\index.xml`).

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

## Update log

- 2026-09-10: Refreshed the placeholder scan from **319** to **301** (reduced by **18**). Related documentation PRs: [#374](https://github.com/dotnet/android-api-docs/pull/374), [#392](https://github.com/dotnet/android-api-docs/pull/392), [#404](https://github.com/dotnet/android-api-docs/pull/404), [#566](https://github.com/dotnet/android-api-docs/pull/566), [#644](https://github.com/dotnet/android-api-docs/pull/644).

Run the provided rg scan, then inspect the matching files under docs\xml\Java.Nio and the ns-Java.Nio.xml files. Replace the 301 “To be added” placeholders with appropriate XML documentation, and rerun the scan to verify that the targeted placeholders are gone.

Escrito por el modelo de indexación a partir del texto del issue.