Clarify that `More` is author-defined in "Layout"?

[Vectornaut]

Reading the example definition

enum Link {
    Empty,
    More(Box<Node>),
}

in the "Basic Data Layout" section, it took me some effort to figure out that More is an author-defined type name, rather than a some kind of Rust built-in. I don't know why this definition confused me, even though the syntactically similar definition

pub enum List {
    Empty,
    Elem(i32, List),
}

didn't, but this forum post shows that at least one other person was confused in the same way. I have about two months of Rust experience, which includes using built-in enums of this form (like Option and Error), but not defining any that I can recall.