fix: explain E0120 better cover cases when its raised

compiler/rustc_error_codes/src/error_codes/E0120.md

@@ -1,7 +1,7 @@
-Drop was implemented on a trait, which is not allowed: only structs and
-enums can implement Drop.
+Drop was implemented on a trait object or reference, which is not allowed; only
+structs, enums, and unions can implement Drop.
 
-Erroneous code example:
+Erroneous code examples:
 
 ```compile_fail,E0120
 trait MyTrait {}
@@ -11,7 +11,15 @@ impl Drop for MyTrait {
 }
 ```
 
-A workaround for this problem is to wrap the trait up in a struct, and implement
+```compile_fail,E0120
+struct Concrete {}
+
+impl Drop for &'_ mut Concrete  {
+    fn drop(&mut self) {}
+}
+```
+
+A workaround for traits is to wrap the trait up in a struct, and implement
 Drop on that:
 
 ```