Add description for initializer dtype
Add description for initializer dtype

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