[docs] [issue29677] clarify docs about 'round()' accepting a negative integer for ndigits

Changes by Mariatta Wijaya <mariatta.wijaya@gmail.com>: ---------- versions: +Python 2.7, Python 3.5, Python 3.6, Python 3.7 _______________________________________ Python tracker <report@bugs.python.org> <http://bugs.python.org/issue29677> _______________________________________