CUDA: Add a note on max_registers kwarg
CUDA: Add a note on max_registers kwarg

The maximum number of registers per thread requested may not be
respected if the ABI requires more registers. This commit adds a small
note about this to the documentation of the `max_registers` kwarg.

numba/numba,stuartarchibald/numba,stuartarchibald/numba,seibert/numba,IntelLabs/numba,cpcloud/numba,stuartarchibald/numba,cpcloud/numba,stuartarchibald/numba,seibert/numba,seibert/numba,cpcloud/numba,gmarkall/numba,IntelLabs/numba,IntelLabs/numba,stonebig/numba,gmarkall/numba,seibert/numba,IntelLabs/numba,stonebig/numba,IntelLabs/numba,stuartarchibald/numba,cpcloud/numba,numba/numba,stonebig/numba,numba/numba,stonebig/numba,cpcloud/numba,numba/numba,gmarkall/numba,stonebig/numba,seibert/numba,gmarkall/numba,numba/numba,gmarkall/numba