Man pages matching name al_is_event_source_registered in section 3alleg5:

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