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).

运行提供的 rg 扫描，然后检查 docs\xml\Java.Nio 下匹配的文件以及 ns-Java.Nio.xml 文件。将 301 个“To be added”占位符替换为适当的 XML 文档，并重新运行扫描以验证目标占位符已被移除。

由索引模型根据 Issue 内容生成。