Add XML documentation for Org.Apache.Http.Util

The `Org.Apache.Http.Util` namespace contains **14** `To be added` placeholders in source XML documentation (excluding `docs\xml\index.xml`).

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

## Update log

- 2026-09-10: Refreshed the placeholder scan from **116** to **14** (reduced by **102**). Related documentation PRs: [#516](https://github.com/dotnet/android-api-docs/pull/516), [#644](https://github.com/dotnet/android-api-docs/pull/644).

Start in docs\xml by running the provided ripgrep command for Org.Apache.Http.Util and the 14 remaining "To be added" placeholders. Update the matching source XML documentation entries, excluding docs\xml\index.xml, then rerun the scan; done means no placeholders remain in the namespace.

Written by the indexing model from the issue text.