Man pages matching name allegro_pixel_format in section 3alleg5:

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