Man pages matching name normalize_vector in section 3alleg4:

normalize_vector.3alleg4.en	debian 10 / allegro4-doc
normalize_vector.3alleg4.en	debian 11 / allegro4-doc
normalize_vector.3alleg4.en	debian 12 / allegro4-doc
normalize_vector.3alleg4.en	debian 13 / allegro4-doc