Add XML documentation for Org.Apache.Http.Message

The `Org.Apache.Http.Message` namespace contains **152** `To be added` placeholders in source XML documentation (excluding `docs\xml\index.xml`).

```powershell
rg -n -i --glob 'Org.Apache.Http.Message/**' --glob 'ns-Org.Apache.Http.Message.xml' 'to be added' docs\xml
```

## Update log

- 2026-09-10: Refreshed the placeholder scan from **322** to **152** (reduced by **170**). Related documentation PRs: [#429](https://github.com/dotnet/android-api-docs/pull/429), [#619](https://github.com/dotnet/android-api-docs/pull/619), [#644](https://github.com/dotnet/android-api-docs/pull/644).

从 Org.Apache.Http.Message 命名空间下的 docs\xml 文件开始，运行提供的 ripgrep 命令来检查剩余的 152 个占位符。替换相关的源 XML 文档条目，但排除 docs\xml\index.xml，然后重新运行扫描；当不再有目标 `To be added` 占位符时即表示完成。

由索引模型根据 Issue 内容生成。