Man pages matching name allegro_shader in section 3alleg5:

ALLEGRO_SHADER.3alleg5.en	debian 10 / allegro5-doc
ALLEGRO_SHADER.3alleg5.en	debian 11 / allegro5-doc
ALLEGRO_SHADER.3alleg5.en	debian 12 / allegro5-doc
ALLEGRO_SHADER.3alleg5.en	debian 13 / allegro5-doc