improve docs of binary_accuracy
improve docs of binary_accuracy

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