Man pages matching name al_create_vertex_decl in section 3alleg5:

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