Clean up E0081 long explanation

src/librustc_error_codes/error_codes/E0081.md

@@ -1,21 +1,23 @@
-Enum discriminants are used to differentiate enum variants stored in memory.
-This error indicates that the same value was used for two or more variants,
-making them impossible to tell apart.
+A discrimant value is present more than once.
+
+Erroneous code example:
 
 ```compile_fail,E0081
-// Bad.
 enum Enum {
     P = 3,
-    X = 3,
+    X = 3, // error!
     Y = 5,
 }
 ```
 
+Enum discriminants are used to differentiate enum variants stored in memory.
+This error indicates that the same value was used for two or more variants,
+making it impossible to distinguish them.
+
 ```
-// Good.
 enum Enum {
     P,
-    X = 3,
+    X = 3, // ok!
     Y = 5,
 }
 ```
@@ -27,7 +29,7 @@ variants.
 ```compile_fail,E0081
 enum Bad {
     X,
-    Y = 0
+    Y = 0, // error!
 }
 ```