Clarify numba ufunc supported features
Clarify numba ufunc supported features

Added a note mentioning that users shouldn't expect all ufunc features to work in compiled code.

Based on comments from https://github.com/numba/numba/issues/4504#issuecomment-527102530.
stonebig/numba,sklam/numba,IntelLabs/numba,stuartarchibald/numba,numba/numba,stonebig/numba,stuartarchibald/numba,gmarkall/numba,IntelLabs/numba,IntelLabs/numba,numba/numba,gmarkall/numba,gmarkall/numba,stonebig/numba,seibert/numba,IntelLabs/numba,stuartarchibald/numba,numba/numba,cpcloud/numba,IntelLabs/numba,cpcloud/numba,sklam/numba,sklam/numba,cpcloud/numba,seibert/numba,stuartarchibald/numba,sklam/numba,gmarkall/numba,sklam/numba,cpcloud/numba,seibert/numba,gmarkall/numba,seibert/numba,stonebig/numba,stonebig/numba,cpcloud/numba,stuartarchibald/numba,seibert/numba,numba/numba,numba/numba