Note why some array methods not implemented (#2351)
Note why some array methods not implemented (#2351)

Add a note to ``array-overview.rst`` that some operations aren't
implemented due to being inefficient for larger datasets.
jakirkham/dask,mrocklin/dask,blaze/dask,mrocklin/dask,mraspaud/dask,dask/dask,jcrist/dask,blaze/dask,jakirkham/dask,jcrist/dask,ContinuumIO/dask,ContinuumIO/dask,mraspaud/dask,dask/dask