Document requirement for IgnoreAllUnexported

[neild]

cpy.New currently requires that the user pass cpy.IgnoreAllUnexported. Document this requirement, or change it.

We require that the user specify this option because it isn't clear what the correct default handling of unexported fields is. (See TODO in the code detailing various possibilities.) If we're confident that there is no natural default and the user will always be required to pick a behavior, perhaps we should promote this option to the function signature:

type Mode int

const (
  IgnoreAllUnexported Mode = iota
)

func New(mode Mode, opts ...Option) *Machine { ... }