Clean up E0199 explanation

src/librustc_error_codes/error_codes/E0199.md

@@ -1,14 +1,23 @@
-Safe traits should not have unsafe implementations, therefore marking an
-implementation for a safe trait unsafe will cause a compiler error. Removing
-the unsafe marker on the trait noted in the error will resolve this problem.
+A trait implementation was marked as unsafe while the trait is safe.
+
+Erroneous code example:
 
 ```compile_fail,E0199
 struct Foo;
 
 trait Bar { }
 
-// this won't compile because Bar is safe
-unsafe impl Bar for Foo { }
-// this will compile
-impl Bar for Foo { }
+unsafe impl Bar for Foo { } // error!
+```
+
+Safe traits should not have unsafe implementations, therefore marking an
+implementation for a safe trait unsafe will cause a compiler error. Removing
+the unsafe marker on the trait noted in the error will resolve this problem:
+
+```
+struct Foo;
+
+trait Bar { }
+
+impl Bar for Foo { } // ok!
 ```