Make note of Ord's derive ordering

Fixes rust-lang#26620

src/libcore/cmp.rs

@@ -166,6 +166,8 @@ impl Ordering {
 ///
 /// - total and antisymmetric: exactly one of `a < b`, `a == b` or `a > b` is true; and
 /// - transitive, `a < b` and `b < c` implies `a < c`. The same must hold for both `==` and `>`.
+///
+/// When this trait is `derive`d, it produces a lexicographic ordering.
 #[stable(feature = "rust1", since = "1.0.0")]
 pub trait Ord: Eq + PartialOrd<Self> {
     /// This method returns an `Ordering` between `self` and `other`.