Improve docs of copy
Improve docs of copy

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