Clarify supported data types for indexing
Clarify supported data types for indexing

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