Improve E0023 long error explanation

Author: GuillaumeGomez

src/librustc_error_codes/error_codes/E0023.md

@@ -2,11 +2,18 @@ A pattern attempted to extract an incorrect number of fields from a variant.
 
 Erroneous code example:
 
-```
+```compile_fail,E0023
 enum Fruit {
     Apple(String, String),
     Pear(u32),
 }
+
+let x = Fruit::Apple(String::new(), String::new());
+
+match x {
+    Fruit::Apple(a) => {}, // error!
+    _ => {}
+}
 ```
 
 A pattern used to match against an enum variant must provide a sub-pattern for