IsConstructedGenericType documentation fix

The [docs ](https://docs.microsoft.com/en-us/dotnet/api/system.type.isconstructedgenerictype?view=netframework-4.8) say "You can create instances of a constructed generic type," but that isn't always true. Consider:

private abstract class MyClass
{
}

This returns `true`

typeof(MyClass).IsConstructedGenericType

This throws:

Activator.CreateInstance(typeof(MyClass));