Add XML documentation for Org.Apache.Http.Client

The `Org.Apache.Http.Client` namespace contains **31** `To be added` placeholders in source XML documentation (excluding `docs\xml\index.xml`).

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

## Update log

- 2026-09-10: Refreshed the placeholder scan from **118** to **31** (reduced by **87**). Related documentation PRs: [#428](https://github.com/dotnet/android-api-docs/pull/428), [#644](https://github.com/dotnet/android-api-docs/pull/644).

Run the provided rg command against docs\xml to list the 31 remaining placeholders under Org.Apache.Http.Client, then inspect the matching source XML documentation entries. Replace each `To be added` placeholder with appropriate documentation and rerun the scan to confirm none remain in the namespace.

Rédigé par le modèle d'indexation à partir du texte de l'issue.