DOC: parallel vectorize requires signatures
DOC: parallel vectorize requires signatures

Related to #1870, previously the docs did not explain that vectorize(target='parallel') won't work at all without signatures specified.
gmarkall/numba,stuartarchibald/numba,gmarkall/numba,stonebig/numba,seibert/numba,sklam/numba,jriehl/numba,IntelLabs/numba,jriehl/numba,IntelLabs/numba,cpcloud/numba,sklam/numba,seibert/numba,stonebig/numba,stuartarchibald/numba,stuartarchibald/numba,stonebig/numba,sklam/numba,stonebig/numba,numba/numba,seibert/numba,IntelLabs/numba,jriehl/numba,stuartarchibald/numba,numba/numba,sklam/numba,seibert/numba,numba/numba,jriehl/numba,gmarkall/numba,cpcloud/numba,stuartarchibald/numba,cpcloud/numba,numba/numba,IntelLabs/numba,sklam/numba,seibert/numba,numba/numba,gmarkall/numba,cpcloud/numba,gmarkall/numba,jriehl/numba,IntelLabs/numba,cpcloud/numba,stonebig/numba