Clean up E0205 explanation

Author: GuillaumeGomez

src/librustc_error_codes/error_codes/E0206.md

@@ -1,12 +1,18 @@
-You can only implement `Copy` for a struct or enum. Both of the following
-examples will fail, because neither `[u8; 256]` nor `&'static mut Bar`
-(mutable reference to `Bar`) is a struct or enum:
+The `Copy` trait was implemented on a type which is neither a struct nor an
+enum.
+
+Erroneous code example:
 
 ```compile_fail,E0206
 type Foo = [u8; 256];
-impl Copy for Foo { } // error
+impl Copy for Foo { } // error!
 
 #[derive(Copy, Clone)]
 struct Bar;
-impl Copy for &'static mut Bar { } // error
+
+impl Copy for &'static mut Bar { } // error!
 ```
+
+You can only implement `Copy` for a struct or an enum. Both of the previous
+examples will fail, because neither `[u8; 256]` nor `&'static mut Bar`
+(mutable reference to `Bar`) is a struct or enum.