The haddocks for `sortWith` should give guidance about when it's prefered over `sortOn`

As ben explained it somewhere else:

In general if the mapping function is expensive to compute then you should probably be using sortOn, as it only needs to compute it once for each element. sortWith, on the other hand must compute the mapping function for every comparison that it performs. Beyond that general intuition it's hard to give any specific prescription.

I think even without more specific guidance stating this much in the docs would be good.