clean up e0200 explanation

src/librustc_error_codes/error_codes/E0200.md

@@ -1,14 +1,23 @@
-Unsafe traits must have unsafe implementations. This error occurs when an
+An unsafe trait was implemented without an unsafe implementation.
-implementation for an unsafe trait isn't marked as unsafe. This may be resolved
+
-by marking the unsafe implementation as unsafe.
+Erroneous code example:
 
 ```compile_fail,E0200
 struct Foo;
 
 unsafe trait Bar { }
 
-// this won't compile because Bar is unsafe and impl isn't unsafe
+impl Bar for Foo { } // error!
-impl Bar for Foo { }
+```
-// this will compile
+
-unsafe impl Bar for Foo { }
+Unsafe traits must have unsafe implementations. This error occurs when an
+implementation for an unsafe trait isn't marked as unsafe. This may be resolved
+by marking the unsafe implementation as unsafe.
+
+```
+struct Foo;
+
+unsafe trait Bar { }
+
+unsafe impl Bar for Foo { } // ok!
 ```