Use forall, & add more comments
Use forall, & add more comments

(code review feedback for #2681)

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