Clean up E0192 explanation

src/librustc_error_codes/error_codes/E0192.md

@@ -1,3 +1,19 @@
+A negative impl was added on a trait implementation.
+
+Erroneous code example:
+
+```compile_fail,E0192
+trait Trait {
+    type Bar;
+}
+
+struct Foo;
+
+impl !Trait for Foo { } //~ ERROR E0192
+
+fn main() {}
+```
+
 Negative impls are only allowed for auto traits. For more
 information see the [opt-in builtin traits RFC][RFC 19].