Fix documentations of negative sampling
Fix documentations of negative sampling

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