Using controlled vocabularies in the admin `list_filter`

[majdal]

Hello!

I'm still using this package extensively in a project. For one of my models, I have a ControlledTermField, and I would like to filter against it in the admin using list_filter.

Current behaviour: If I add my ControlledTermField in my list_filter, all the controlled terms from all the vocabularies are listed in the filter, not just the ones from that specific field.

Desired behaviour: only the terms that belong to the vocabulary of the specified field should be displayed.

According to the Django docs, we can set a SimpleListFilter, and that works perfectly fine. I'm wondering two things:

1. If there is a way to abstract the SimpleListFilter and include it in the package
2. If abstraction would be too cumbersome/unnecessary, perhaps include a snippet in the docs to guide future users of the package.

Thanks again for the great work on the package.