derive explanation for PartialOrd should match that for Ord

I think these just got out of sync, but both use a lexicographic
ordering.

Relevant commits in the history of these explanations:
* 8b81f76 on 2015-06-30
* e22770b on 2016-02-09

src/libcore/cmp.rs

@@ -254,8 +254,8 @@ fn partial_cmp(&self, other: &Ordering) -> Option<Ordering> {
 /// total order. For example, for floating point numbers, `NaN < 0 == false` and `NaN >= 0 ==
 /// false` (cf. IEEE 754-2008 section 5.11).
 ///
-/// This trait can be used with `#[derive]`. When `derive`d, it will produce an ordering
-/// based on the top-to-bottom declaration order of the struct's members.
+/// This trait can be used with `#[derive]`. When `derive`d, it will produce a lexicographic
+/// ordering based on the top-to-bottom declaration order of the struct's members.
 ///
 /// # Examples
 ///