Update upgrading guide about error handling in transactional callbacks

- Part of #16576
- [ci skip]
Maybe something like...

```
When you define a `after_rollback` or `after_commit` callback, you will receive a deprecation
warning about this upcoming change. When you are ready, you can opt into the new behavior
and remove the warning by adding the following configuration to your `config/application.rb`:

   config.active_record.raise_in_transactional_callbacks = true

See #14488 and #16537 for background.
```

(feel free to iterate and improve upon that initial suggestion :smile:)