That documentation doesn't answer the questions does it?
There is no way to do this I know of in stock phpBB. It would need an extension or custom coding.
Using phpMyadmin in your hosting control panel open phpbb_attachments table. Click the search tab and next to is_orphan type 1 and hit enter. The poster_id is the id of the poster.
“Results! Why, man, I have gotten a lot of results! I have found several thousand things that won’t work.”