Add note that random forest restricts variables on a node level

[tompollard]

Make it clear that the random forest is restricting the variables available at each decision in the node (rather than restricting which variables are available for training of the entire tree): https://carpentries-incubator.github.io/machine-learning-trees-python/06-random-forest/

[tompollard]

Perhaps also add an exercise that asks what we expect to see if the max_depth is set to 1 (we expect each tree to split on only one of the two variables).