union filter documented incorrectly

[agowa]

The invalid documentation of union was fixed in ansible/ansible#82199 but didn't make it into this section of the docs.

I also quickly ran the other set filters through ansible as well to match their documentation to the implementation

[ansible-documentation-bot]

Thanks for your Ansible docs contribution! We talk about Ansible documentation on Matrix at #docs:ansible.im if you ever want to join us and chat about the docs! We meet on Matrix every Tuesday. See the Ansible calendar for meeting details. We welcome additions to our weekly agenda items too. You can add the dawgs-meeting tag to a forum topic to bring it up at the next meeting.