Documentation of 'Property of class preceded by question mark'

Documentation of 'Property of class preceded by question mark'

I have class that uses a ulong ParentId { get; set; }and I needed it to be
nullable. Because if the entity (of this class) is added to the database,
and has no parent, it must be null and not 0. So I remembered vaguely
something about preceding it with a question mark. And it works. See the
code below.
class ExampleClass
{
public ulong ?ParentId { get; set; }
}
Question: Does anyone know where to find the documentation for this?