[docs] [issue18697] Unify arguments names in Unicode object C API documentation

Change by Stéphane Wirtel <stephane@wirtel.be>: ---------- versions: +Python 3.8, Python 3.9 -Python 3.4, Python 3.5, Python 3.6 _______________________________________ Python tracker <report@bugs.python.org> <https://bugs.python.org/issue18697> _______________________________________