Make note that Ordering is builtin.

This way people won't try to copy/paste it in.

Author: steveklabnik

src/doc/guide.md

@@ -1073,20 +1073,19 @@ destructuring `let`.
 ## Enums
 
 Finally, Rust has a "sum type", an **enum**. Enums are an incredibly useful
-feature of Rust, and are used throughout the standard library. Enums look
-like this:
+feature of Rust, and are used throughout the standard library. This is an enum
+that is provided by the Rust standard library:
 
-```
+```{rust}
 enum Ordering {
     Less,
     Equal,
     Greater,
 }
 ```
 
-This is an enum that is provided by the Rust standard library. An `Ordering`
-can only be _one_ of `Less`, `Equal`, or `Greater` at any given time. Here's
-an example:
+An `Ordering` can only be _one_ of `Less`, `Equal`, or `Greater` at any given
+time. Here's an example:
 
 ```rust
 fn cmp(a: int, b: int) -> Ordering {