Add XML documentation for Org.Apache.Http.Cookies

The `Org.Apache.Http.Cookies` namespace contains **83** `To be added` placeholders in source XML documentation (excluding `docs\xml\index.xml`).

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

## Update log

- 2026-09-10: Refreshed the placeholder scan from **86** to **83** (reduced by **3**). Related documentation PRs: [#556](https://github.com/dotnet/android-api-docs/pull/556), [#644](https://github.com/dotnet/android-api-docs/pull/644).

首先针对 docs\xml 运行提供的 rg 命令，以确认当前 83 个占位符的范围，并排除 docs\xml\index.xml。阅读 Org.Apache.Http.Cookies 源 XML 文档中匹配的条目，并将每个“To be added”占位符替换为准确的文档。范围扫描未发现剩余占位符即表示完成。

由索引模型根据 Issue 内容生成。