title: How to use update_fields in Django signals
body: <p>I'm trying to use an the 'update_fields' argument passed to signals in Django.</p>
  
  <p>I've got something very simple for now, namely:</p>
  
  <pre><code>@receiver(pre_save, sender=models.UserAdmin)
  @receiver(pre_save, sender=models.UserGroupAdmin)
  def update_timestamps(sender, instance, update_fields, **kwargs):
      print(f'Update fields: {update_fields}')
  </code></pre>
  
  <p><code>update_fields</code> shows as None no matter what is being updated, which indicates I've not understood something.</p>
  
  <p>What am I missing?</p>