Man pages matching name al_ustr_rtrim_ws in section 3alleg5:

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