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.

Written by the indexing model from the issue text.