Add more explanation to MNIST example comments
Add more explanation to MNIST example comments

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