Fix document of MemoryPointer.copy_from_device and copy_from_device_async
Fix document of MemoryPointer.copy_from_device and copy_from_device_async

cupy/cupy,keisuke-umezawa/chainer,kikusu/chainer,jnishi/chainer,t-abe/chainer,kikusu/chainer,hvy/chainer,jnishi/chainer,benob/chainer,delta2323/chainer,okuta/chainer,AlpacaDB/chainer,cupy/cupy,okuta/chainer,sinhrks/chainer,niboshi/chainer,muupan/chainer,chainer/chainer,cupy/cupy,sinhrks/chainer,jnishi/chainer,wkentaro/chainer,ktnyt/chainer,muupan/chainer,keisuke-umezawa/chainer,ktnyt/chainer,chainer/chainer,aonotas/chainer,wkentaro/chainer,hvy/chainer,rezoo/chainer,okuta/chainer,cemoody/chainer,jnishi/chainer,ysekky/chainer,hvy/chainer,wkentaro/chainer,cupy/cupy,truongdq/chainer,chainer/chainer,wkentaro/chainer,ktnyt/chainer,tscohen/chainer,okuta/chainer,niboshi/chainer,ronekko/chainer,niboshi/chainer,tkerola/chainer,ktnyt/chainer,minhpqn/chainer,keisuke-umezawa/chainer,chainer/chainer,anaruse/chainer,t-abe/chainer,kiyukuta/chainer,hvy/chainer,truongdq/chainer,pfnet/chainer,niboshi/chainer,AlpacaDB/chainer,keisuke-umezawa/chainer,benob/chainer,kashif/chainer