[docs] [issue15355] generator.__next__() docs should mention exception if already executing

Chris Jerdonek added the comment: Terry, if when reviewing my patch for issue 15457, you also have time to review this much simpler patch (also in the documentation on generators), I would appreciate it. If not, that's okay. ---------- nosy: +terry.reedy _______________________________________ Python tracker <report@bugs.python.org> <http://bugs.python.org/issue15355> _______________________________________