text: Title: Documentation for customizing admin forms is out-of-date / unclear
  Body: The documentation describes how to [customize the import forms](https://django-import-export.readthedocs.io/en/latest/getting_started.html#customize-admin-import-forms).  The example shows how you can add an `author` field, but it doesn't make it clear what the purpose of this is, and that you have to provide any filtering capabilities yourself.  This has led to some [confusion](https://stackoverflow.com/q/71161607/39296).
  
  I propose to update this section so that it is clearer why you would want to customize the import forms, and to give an example of how to filter using a dropdown.