Debug.Assert does not open message box.

The documentation for Debug.Assert says that it opens a message box. However this capability was removed in .NET Core 3.1, according to this issue: https://github.com/dotnet/winforms/issues/3739

It's not clear from the discussion there that there is a plan to reintroduce the message box behavior, so in the meantime it would make sense to make the docs consistent with the shipped behavior of dotnet.