Support None for get_trigger and make the docstring clear
Support None for get_trigger and make the docstring clear

wkentaro/chainer,kashif/chainer,tkerola/chainer,keisuke-umezawa/chainer,hvy/chainer,wkentaro/chainer,pfnet/chainer,wkentaro/chainer,hvy/chainer,chainer/chainer,niboshi/chainer,hvy/chainer,niboshi/chainer,keisuke-umezawa/chainer,kikusu/chainer,keisuke-umezawa/chainer,hvy/chainer,kiyukuta/chainer,okuta/chainer,jnishi/chainer,cupy/cupy,jnishi/chainer,ysekky/chainer,delta2323/chainer,okuta/chainer,ktnyt/chainer,ktnyt/chainer,kikusu/chainer,chainer/chainer,ronekko/chainer,chainer/chainer,keisuke-umezawa/chainer,anaruse/chainer,ktnyt/chainer,okuta/chainer,niboshi/chainer,wkentaro/chainer,ktnyt/chainer,jnishi/chainer,cupy/cupy,chainer/chainer,niboshi/chainer,cupy/cupy,aonotas/chainer,jnishi/chainer,okuta/chainer,cupy/cupy,rezoo/chainer