Add XML documentation for Org.Apache.Http.Impl.Client

The `Org.Apache.Http.Impl.Client` namespace contains **411** `To be added` placeholders in source XML documentation (excluding `docs\xml\index.xml`).

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

## Update log

- 2026-09-10: Refreshed the placeholder scan from **599** to **411** (reduced by **188**). Related documentation PRs: [#517](https://github.com/dotnet/android-api-docs/pull/517), [#598](https://github.com/dotnet/android-api-docs/pull/598), [#644](https://github.com/dotnet/android-api-docs/pull/644).

Start by running the provided rg command against docs\xml to enumerate the 411 placeholders under Org.Apache.Http.Impl.Client, then inspect the matching XML files and nearby completed entries for context. Replace the placeholders with accurate XML documentation and rerun the scan; done means the targeted placeholder count is reduced or eliminated without changing docs\xml\index.xml.

Written by the indexing model from the issue text.