Rollup merge of #42920 - behnam:cmp, r=BurntSushi

[libcore/cmp] Expand Ord/PartialOrd Derivable doc for enum types

Expand Derivable docblock section for `Ord` and `PartialOrd` to cover
`enum` types, in addition to the existing language explaining it for
`struct` types.

Author: Ariel Ben-Yehuda

src/libcore/cmp.rs

@@ -379,8 +379,9 @@ impl<T: Ord> Ord for Reverse<T> {
 ///
 /// ## Derivable
 ///
-/// 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.
+/// This trait can be used with `#[derive]`. When `derive`d on structs, it will produce a
+/// lexicographic ordering based on the top-to-bottom declaration order of the struct's members.
+/// When `derive`d on enums, variants are ordered by their top-to-bottom declaration order.
 ///
 /// ## How can I implement `Ord`?
 ///
@@ -512,8 +513,9 @@ impl PartialOrd for Ordering {
 ///
 /// ## Derivable
 ///
-/// 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.
+/// This trait can be used with `#[derive]`. When `derive`d on structs, it will produce a
+/// lexicographic ordering based on the top-to-bottom declaration order of the struct's members.
+/// When `derive`d on enums, variants are ordered by their top-to-bottom declaration order.
 ///
 /// ## How can I implement `PartialOrd`?
 ///