Add XML documentation for Org.Apache.Http.Conn.Schemes

The `Org.Apache.Http.Conn.Schemes` namespace contains **26** `To be added` placeholders in source XML documentation (excluding `docs\xml\index.xml`).

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

## Update log

- 2026-09-10: Refreshed the placeholder scan from **50** to **26** (reduced by **24**). Related documentation PRs: [#535](https://github.com/dotnet/android-api-docs/pull/535), [#644](https://github.com/dotnet/android-api-docs/pull/644).

Run the provided rg command from the repository root to list the 26 remaining placeholders under docs\xml\Org.Apache.Http.Conn.Schemes and ns-Org.Apache.Http.Conn.Schemes.xml. Review those XML documentation entries, replace each “To be added” placeholder with the appropriate documentation, then rerun the scan; done means no matching placeholders remain.

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