Document the implementation of #[derive(Ord)]

[anmej]

(0, (1,0)) < (1, (0,1)) but ((1,0), 0) > ((0,1), 1)
The order of the fields is important, and this is something that should be clearly documented, as changing the order of the fields of a struct can change program logic.

[sfackler]

#[derive(Ord)] generates a lexicographic ordering.