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

The `Org.Apache.Http.Client.Methods` namespace contains **4** `To be added` placeholders in source XML documentation (excluding `docs\xml\index.xml`).

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

## Update log

- 2026-09-10: Refreshed the placeholder scan from **56** to **4** (reduced by **52**). Related documentation PRs: [#589](https://github.com/dotnet/android-api-docs/pull/589), [#644](https://github.com/dotnet/android-api-docs/pull/644).

Run the provided rg command in docs\xml to locate the four remaining “To be added” placeholders under Org.Apache.Http.Client.Methods, excluding docs\xml\index.xml. Read the surrounding XML and related API documentation to determine the missing descriptions. Done means all four placeholders in the namespace documentation are replaced with accurate text and the scan no longer reports them.

由索引模型根据 Issue 内容生成。