Cleanup E0391 explanation

Author: GuillaumeGomez

src/librustc_error_codes/error_codes/E0391.md

@@ -1,7 +1,6 @@
-This error indicates that some types or traits depend on each other
-and therefore cannot be constructed.
+A type dependency cycle has been encountered.
 
-The following example contains a circular dependency between two traits:
+Erroneous code example:
 
 ```compile_fail,E0391
 trait FirstTrait : SecondTrait {
@@ -12,3 +11,6 @@ trait SecondTrait : FirstTrait {
 
 }
 ```
+
+The previous example contains a circular dependency between two traits:
+`FirstTrait` depends on `SecondTrait` which itself depends on `FirstTrait`.