Man pages matching name allegro_event_type_is_user in section 3alleg5:

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