Fix documentation for mixed16 mode
Fix documentation for mixed16 mode

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