Man pages matching name set_write_alpha_blender in section 3alleg4:

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