Modreq usage of `UnmanagedType` type should be documented

**Issue description**

The code following C# code
```cs
public void UnmanagedT() where T : unmanaged { }
```
generates the following IL code:
```cil
.method public hidebysig
instance void UnmanagedT () 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**

- [x] .NET Core 1.0+
- [x] .NET Framework 1.1+
- [x] .NET Standard 1.1+