improve docs of softplus
improve docs of softplus

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