Man pages matching name al_convert_mask_to_alpha in section 3alleg5:

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