add a doc saying that behavior is the same as OpenCV
add a doc saying that behavior is the same as OpenCV

hvy/chainer,wkentaro/chainer,wkentaro/chainer,hvy/chainer,okuta/chainer,okuta/chainer,wkentaro/chainer,chainer/chainer,wkentaro/chainer,chainer/chainer,niboshi/chainer,niboshi/chainer,hvy/chainer,okuta/chainer,chainer/chainer,chainer/chainer,pfnet/chainer,niboshi/chainer,okuta/chainer,hvy/chainer,niboshi/chainer