Modreq usage of `UnmanagedType` type should be documented

Issue description

The code following C# code

public void UnmanagedT<T>() where T : unmanaged { }

generates the following IL code:

.method public hidebysig 
instance void UnmanagedT<valuetype .ctor (class [mscorlib]System.ValueType modreq([mscorlib]System.Runtime.InteropServices.UnmanagedType)) T> () cil managed 
{
.param type T
    .custom instance void System.Runtime.CompilerServices.IsUnmanagedAttribute::.ctor() = (
        01 00 00 00
    )
.maxstack 8
IL_0000: ret
}

which expect any compilers wanting to use the method to know that the type parameter T has to meet certain requirements.

With the System.Runtime.CompilerServices.IsVolatile type it is documented how & what the modreq does, it should be the same here. It would also be good if System.Runtime.CompilerServices.IsExternalInit (and any others missing it) got the same documentation.

I'm looking on https://docs.microsoft.com/en-us/dotnet/api/system.runtime.interopservices.unmanagedtype?view=net-6.0.

Target framework

• .NET Core 1.0+
• .NET Framework 1.1+
• .NET Standard 1.1+

Inizia dalla documentazione API collegata di UnmanagedType e confrontala con la documentazione esistente di System.Runtime.CompilerServices.IsVolatile. Documenta in che modo modreq comunica il requisito del parametro di tipo unmanaged e verifica se anche IsExternalInit o altri tipi correlati necessitano di una documentazione equivalente; il lavoro è completato quando i riferimenti API pertinenti spiegano questo comportamento.