Improve docs of expand_dims
Improve docs of expand_dims

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