Document new support for axis in argmax.
Document new support for axis in argmax.

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