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.

索引モデルが issue の本文から書いたものです。