Mention that train_test_split returns a random split

I saw https://github.com/scikit-learn/scikit-learn/commit/17bca23b455f45c0b28d3ff04b05252317c76552 - cudos for adding this note! Shuffling always bothered me, and it is an issue not only for cross-validation, but also for `train_test_split` (which shuffles by default) - maybe mention it as well?

Sorry for a bad diff; I've removed improper indentation. What is changed is "Keep in mind that :func:`train_test_split` still returns a random split." sentence added to the second point.