Clarify differences in boolean indexing between dask and numpy arrays (#6764)
Clarify differences in boolean indexing between dask and numpy arrays (#6764)

* Hint how to do boolean indexing

dask does boolean indexing differently compared to numpy, add a hint how it's done in the error. 
Make if-condition a little easier to read.

* Undo condition changes

* Move error tot setitem instead.
jakirkham/dask,dask/dask,jakirkham/dask,blaze/dask,blaze/dask,dask/dask