Improve the Extension docstring
Improve the Extension docstring

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