Add note that random forest restricts variables on a node level

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/

Start with the random forest section at the linked lesson page and read the explanation of how variables are selected. Revise the note so it clearly says variables are restricted at each node or decision, not for training the entire tree; the lesson text should directly reflect that distinction.

Written by the indexing model from the issue text.