Add XML documentation for Javax.Xml.Transform

The `Javax.Xml.Transform` namespace contains **17** `To be added` placeholders in source XML documentation (excluding `docs\xml\index.xml`).

```powershell
rg -n -i --glob 'Javax.Xml.Transform/**' --glob 'ns-Javax.Xml.Transform.xml' 'to be added' docs\xml
```

## Update log

- 2026-09-10: Refreshed the placeholder scan from **51** to **17** (reduced by **34**). Related documentation PRs: [#414](https://github.com/dotnet/android-api-docs/pull/414), [#644](https://github.com/dotnet/android-api-docs/pull/644).

Start with docs\xml\Javax.Xml.Transform and ns-Javax.Xml.Transform.xml, using the provided rg command to locate the 17 remaining "To be added" placeholders. Replace those source XML documentation placeholders with appropriate text, then rerun the scan; done means no matching placeholders remain in the targeted files, excluding docs\xml\index.xml.

索引モデルが issue の本文から書いたものです。