Add XML documentation for Org.Apache.Http.Authentication

The `Org.Apache.Http.Authentication` namespace contains **101** `To be added` placeholders in source XML documentation (excluding `docs\xml\index.xml`).

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

## Update log

- 2026-09-10: Refreshed the placeholder scan from **93** to **101** (increased by **8**). Related documentation PRs: [#644](https://github.com/dotnet/android-api-docs/pull/644).

Start with the provided rg command and inspect the matching XML documentation under docs\xml, including the Org.Apache.Http.Authentication paths and ns-Org.Apache.Http.Authentication.xml. Replace the 101 “To be added” placeholders with appropriate documentation, excluding docs\xml\index.xml, then rerun the scan to verify no placeholders remain.

Vom Indexierungsmodell aus dem Issue-Text verfasst.