Loo comparison in reference to standard error

[jonah]

@avehtari

The problem is that the current SE estimate is optimistic, and the theory say that it in the worst case the true SE can be even twice the estimate, so based on that 4SE using the current estimate is safe

Yeah. I think we can be more explicit about that in the vignettes and maybe other places in the doc. I think all we have right now is in the doc for compare. I’ll open an issue for this.