improve FunctionNode upgrade guide
improve FunctionNode upgrade guide

wkentaro/chainer,jnishi/chainer,ktnyt/chainer,niboshi/chainer,ktnyt/chainer,keisuke-umezawa/chainer,hvy/chainer,ktnyt/chainer,niboshi/chainer,hvy/chainer,okuta/chainer,wkentaro/chainer,chainer/chainer,wkentaro/chainer,chainer/chainer,niboshi/chainer,okuta/chainer,niboshi/chainer,keisuke-umezawa/chainer,tkerola/chainer,hvy/chainer,chainer/chainer,chainer/chainer,jnishi/chainer,jnishi/chainer,ktnyt/chainer,keisuke-umezawa/chainer,pfnet/chainer,wkentaro/chainer,okuta/chainer,jnishi/chainer,hvy/chainer,okuta/chainer,keisuke-umezawa/chainer