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

Changes by Brett Cannon <brett@python.org>: ---------- title: 'round()' accepts a negative integer for ndigits -> clarify docs about 'round()' accepting a negative integer for ndigits _______________________________________ Python tracker <report@bugs.python.org> <http://bugs.python.org/issue29677> _______________________________________