update docstring in convolution_2d.py
update docstring in convolution_2d.py

niboshi/chainer,ronekko/chainer,hvy/chainer,ysekky/chainer,tkerola/chainer,ktnyt/chainer,wkentaro/chainer,chainer/chainer,ktnyt/chainer,wkentaro/chainer,keisuke-umezawa/chainer,wkentaro/chainer,keisuke-umezawa/chainer,ktnyt/chainer,niboshi/chainer,keisuke-umezawa/chainer,cupy/cupy,chainer/chainer,jnishi/chainer,cupy/cupy,hvy/chainer,okuta/chainer,keisuke-umezawa/chainer,niboshi/chainer,delta2323/chainer,jnishi/chainer,chainer/chainer,kiyukuta/chainer,aonotas/chainer,jnishi/chainer,jnishi/chainer,pfnet/chainer,okuta/chainer,niboshi/chainer,cupy/cupy,anaruse/chainer,chainer/chainer,wkentaro/chainer,kashif/chainer,okuta/chainer,hvy/chainer,rezoo/chainer,hvy/chainer,ktnyt/chainer,okuta/chainer,cupy/cupy