Add "n" kwarg, and return value docs for FFTs.
Add "n" kwarg, and return value docs for FFTs.

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