Add XML documentation for Org.Apache.Http.Impl.Entity

The `Org.Apache.Http.Impl.Entity` namespace contains **40** `To be added` placeholders in source XML documentation (excluding `docs\xml\index.xml`).

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

## Update log

- 2026-09-10: Refreshed the placeholder scan from **75** to **40** (reduced by **35**). Related documentation PRs: [#523](https://github.com/dotnet/android-api-docs/pull/523), [#525](https://github.com/dotnet/android-api-docs/pull/525), [#644](https://github.com/dotnet/android-api-docs/pull/644).

针对 docs\xml 运行提供的 rg 命令，检查 Org.Apache.Http.Impl.Entity 下剩余的 40 个 `To be added` 占位符，并排除 docs\xml\index.xml。将这些占位符替换为 XML 文档，然后重新运行扫描，以确认目标 namespace 中不再有任何占位符。

由索引模型根据 Issue 内容生成。