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).

Run the provided rg command against docs\xml to inspect the 40 remaining `To be added` placeholders under Org.Apache.Http.Impl.Entity, excluding docs\xml\index.xml. Replace those placeholders with XML documentation, then rerun the scan to confirm that none remain in the targeted namespace.

Vom Indexierungsmodell aus dem Issue-Text verfasst.