Add `backend.get_device` and `backend.using_device` to doc
Add `backend.get_device` and `backend.using_device` to doc

niboshi/chainer,okuta/chainer,chainer/chainer,okuta/chainer,chainer/chainer,wkentaro/chainer,tkerola/chainer,hvy/chainer,niboshi/chainer,okuta/chainer,keisuke-umezawa/chainer,keisuke-umezawa/chainer,niboshi/chainer,hvy/chainer,pfnet/chainer,wkentaro/chainer,hvy/chainer,wkentaro/chainer,okuta/chainer,keisuke-umezawa/chainer,keisuke-umezawa/chainer,wkentaro/chainer,chainer/chainer,chainer/chainer,niboshi/chainer,hvy/chainer