add numexpr nthreads comment in docstring
add numexpr nthreads comment in docstring

cpcloud/dask,jayhetee/dask,ssanderson/dask,mraspaud/dask,ContinuumIO/dask,PhE/dask,ssanderson/dask,wiso/dask,ContinuumIO/dask,dask/dask,jcrist/dask,vikhyat/dask,pombredanne/dask,gameduell/dask,mikegraham/dask,vikhyat/dask,mrocklin/dask,wiso/dask,dask/dask,jakirkham/dask,jayhetee/dask,clarkfitzg/dask,blaze/dask,clarkfitzg/dask,mraspaud/dask,pombredanne/dask,mrocklin/dask,jakirkham/dask,jcrist/dask,cowlicks/dask,blaze/dask,chrisbarber/dask,PhE/dask