Clean up E0515 explanation

Author: GuillaumeGomez

src/librustc_error_codes/error_codes/E0515.md

@@ -1,7 +1,4 @@
-Cannot return value that references local variable
-
-Local variables, function parameters and temporaries are all dropped before the
-end of the function body. So a reference to them cannot be returned.
+A reference to a local variable was returned.
 
 Erroneous code example:
 
@@ -20,6 +17,9 @@ fn get_dangling_iterator<'a>() -> Iter<'a, i32> {
 }
 ```
 
+Local variables, function parameters and temporaries are all dropped before the
+end of the function body. So a reference to them cannot be returned.
+
 Consider returning an owned value instead:
 
 ```