E0206 - added union to description

compiler/rustc_error_codes/src/error_codes/E0206.md

@@ -1,5 +1,5 @@
 The `Copy` trait was implemented on a type which is neither a struct nor an
-enum.
+enum nor a union.
 
 Erroneous code example:
 
@@ -10,6 +10,6 @@ struct Bar;
 impl Copy for &'static mut Bar { } // error!
 ```
 
-You can only implement `Copy` for a struct or an enum.
+You can only implement `Copy` for a struct, a union, or an enum.
 The previous example will fail because `&'static mut Bar`
 is not a struct or enum.