Man pages matching name al_unregister_event_source in section 3alleg5:

al_unregister_event_source.3alleg5.en	debian 10 / allegro5-doc
al_unregister_event_source.3alleg5.en	debian 11 / allegro5-doc
al_unregister_event_source.3alleg5.en	debian 12 / allegro5-doc