[docs] [issue35880] math.sin has no backward error; this isn't documented

Change by Terry J. Reedy <tjreedy@udel.edu>: ---------- stage: -> needs patch versions: -Python 3.4, Python 3.5, Python 3.6 _______________________________________ Python tracker <report@bugs.python.org> <https://bugs.python.org/issue35880> _______________________________________