Provide more definitive guidance on the use of `run_par()`

[TheLostLambda]

This seems to imply it does speed things up: https://www.reddit.com/r/rust/comments/ij0ost/comment/g3ilyqj/?utm_source=share&utm_medium=web3x&utm_name=web3xcss&utm_term=1&utm_content=share_button

But I don't know if that's the current implementation?