Man pages matching name al_ustr_rfind_chr in section 3alleg5:

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