Add a note about how to permute with replacement

[CraftSpider]

Currently, trying to get 'permutations with replacement', it's hard to find in the documentation how to do that. I eventually found on the github from the issue about adding permutations how repeat_n(iter, iter.len()).multi_cartesian_product() can do what I want, but it would be nice if this was mentioned in the docs, either on permutations or as a search alias on multi_cartesian_product.

Alternatively, it might be nice to have permutation_with_replacement, but I'm not sure if that's common enough that it's desired in the API.

[Raizex]

This was exactly what I needed as well, and was no where near obvious on how to achieve.

This is a very common use case, I would imagine, as it's a necessary step for computing the probability of rolling sum s on n dice and other similar probability calculations.