Add XML documentation for Java.Net

The `Java.Net` namespace contains **278** `To be added` placeholders in source XML documentation (excluding `docs\xml\index.xml`).

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

## Update log

- 2026-09-10: Refreshed the placeholder scan from **333** to **278** (reduced by **55**). Related documentation PRs: [#368](https://github.com/dotnet/android-api-docs/pull/368), [#389](https://github.com/dotnet/android-api-docs/pull/389), [#395](https://github.com/dotnet/android-api-docs/pull/395), [#565](https://github.com/dotnet/android-api-docs/pull/565), [#628](https://github.com/dotnet/android-api-docs/pull/628), [#644](https://github.com/dotnet/android-api-docs/pull/644).

Start with the documented scan command over docs\xml, especially the Java.Net/** files and ns-Java.Net.xml, and review the existing Java.Net entries to understand the expected XML documentation style. Replace the 278 `To be added` placeholders with appropriate documentation, then rerun the scan to confirm no placeholders remain in this scope.

由索引模型根据 Issue 内容生成。