clarify difference between Optimizer.update and Updater.update by differentiating them.
clarify difference between Optimizer.update and Updater.update by differentiating them.

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