Clean up E0371 explanation

Author: GuillaumeGomez

src/librustc_error_codes/error_codes/E0371.md

@@ -1,9 +1,6 @@
-When `Trait2` is a subtrait of `Trait1` (for example, when `Trait2` has a
-definition like `trait Trait2: Trait1 { ... }`), it is not allowed to implement
-`Trait1` for `Trait2`. This is because `Trait2` already implements `Trait1` by
-definition, so it is not useful to do this.
+A trait was implemented on another which already automatically implemented it.
 
-Example:
+Erroneous code examples:
 
 ```compile_fail,E0371
 trait Foo { fn foo(&self) { } }
@@ -15,3 +12,8 @@ impl Foo for Baz { } // error, `Baz` implements `Bar` which implements `Foo`
 impl Baz for Baz { } // error, `Baz` (trivially) implements `Baz`
 impl Baz for Bar { } // Note: This is OK
 ```
+
+When `Trait2` is a subtrait of `Trait1` (for example, when `Trait2` has a
+definition like `trait Trait2: Trait1 { ... }`), it is not allowed to implement
+`Trait1` for `Trait2`. This is because `Trait2` already implements `Trait1` by
+definition, so it is not useful to do this.