Better document/test ordering constraints

[kupiakos]

This runs the derive on the enum definition and not the generated struct:

#[derive(IntoBytes, FromBytes, Immutable)]
#[open_enum]
#[repr(u32)]
pub enum TestEnum {
    FirstValue = 0xaabbccdd_u32.to_be(),
}

Switching the open_enum attribute to the top causes it to process the struct as expected. The zerocopy maintainers are already aware of the general issue. Fortunately this can never cause soundness problems as open-enum always generates types less strict than their equivalent enums.