> So I put all this into a new internal helper function _get_update_rootps_whitening(), and expanded the description a little bit to be more specific about the meaning of rootps and why the final power spectrum is flat. I'm happy to add more, so do let me know what you think!

Looks good.  Mostly the (obvious in retrospect, but nonetheless eluded me even when I was thinking about it) point that rootps means sqrt(ps) was what I needed.  So the inline docs are quite sufficient now. Thank you.