HWNDs do not render if WPF Window's AllowsTransparency is true

Wasted days on this, but if your main WPF Window sets AllowsTransparency, the HWND you create and use under this class will never render anything. Deserves a warning either here, or in the linked "WPF and Win32 Interop" article.

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* ID: 98ca0b90-9c25-12ff-4fa3-49a35ed32d61
* Version Independent ID: 678f7e0d-ce5a-5400-e7db-ef6d1de8592e
* Content: [HwndHost Class (System.Windows.Interop)](https://docs.microsoft.com/en-us/dotnet/api/system.windows.interop.hwndhost?view=netframework-4.7.1)
* Content Source: [xml/System.Windows.Interop/HwndHost.xml](https://github.com/dotnet/dotnet-api-docs/blob/master/xml/System.Windows.Interop/HwndHost.xml)
* Product: **.net**
* GitHub Login: @dotnet-bot
* Microsoft Alias: **dotnetcontent**