Add XML documentation for Org.Apache.Http.Message

The `Org.Apache.Http.Message` namespace contains **152** `To be added` placeholders in source XML documentation (excluding `docs\xml\index.xml`).

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

## Update log

- 2026-09-10: Refreshed the placeholder scan from **322** to **152** (reduced by **170**). Related documentation PRs: [#429](https://github.com/dotnet/android-api-docs/pull/429), [#619](https://github.com/dotnet/android-api-docs/pull/619), [#644](https://github.com/dotnet/android-api-docs/pull/644).

Start with the docs\xml files under the Org.Apache.Http.Message namespace and run the provided ripgrep command to review the 152 remaining placeholders. Replace the relevant source XML documentation entries, excluding docs\xml\index.xml, and rerun the scan; done means no targeted `To be added` placeholders remain.

Vom Indexierungsmodell aus dem Issue-Text verfasst.