Improve docs of huber_loss
Improve docs of huber_loss

niboshi/chainer,keisuke-umezawa/chainer,ktnyt/chainer,keisuke-umezawa/chainer,hvy/chainer,rezoo/chainer,wkentaro/chainer,chainer/chainer,chainer/chainer,niboshi/chainer,jnishi/chainer,aonotas/chainer,chainer/chainer,keisuke-umezawa/chainer,wkentaro/chainer,niboshi/chainer,jnishi/chainer,ktnyt/chainer,jnishi/chainer,wkentaro/chainer,wkentaro/chainer,ronekko/chainer,anaruse/chainer,keisuke-umezawa/chainer,ktnyt/chainer,niboshi/chainer,tkerola/chainer,chainer/chainer,okuta/chainer,hvy/chainer,pfnet/chainer,ktnyt/chainer,hvy/chainer,okuta/chainer,jnishi/chainer,okuta/chainer,hvy/chainer,okuta/chainer