Changed special ndarray subtype attr to __numba_array_subtype_dispatch__.  Added long comment explaining it is experimental and how to use it.
Changed special ndarray subtype attr to __numba_array_subtype_dispatch__.  Added long comment explaining it is experimental and how to use it.

cpcloud/numba,seibert/numba,cpcloud/numba,seibert/numba,numba/numba,IntelLabs/numba,IntelLabs/numba,numba/numba,IntelLabs/numba,numba/numba,cpcloud/numba,cpcloud/numba,IntelLabs/numba,seibert/numba,cpcloud/numba,numba/numba,numba/numba,IntelLabs/numba,seibert/numba,seibert/numba