[resampling] Properties hold-out | cv | loocv

[ludwigbothmann]

It seems to be unclear to some students what the advantages and disadvantages of hold-out | cv | loo-cv are. We should formalize these better. For example, some students said in the exam that a disadvantage of hold-out is the large variance, and in the very next sentence that a disadvantage of loo-cv is also the large variance.

[berndbischl]

thats actually true for both methods, so LOO and holdout, but with different levels of "badness"

[lisa-wm]

related to #994, #611