DOC Detail on parent-child relationship in tree

Some explanation of the `Tree` object seemed lacking (especially as the class docstream boasts that "you can find a detailed description of all arrays below"!). It seems to me that the parent index always precedes its children and that the left child is the `x <= t` case and right is `x > t`.  But I need to confirm that these are true, and that I've described them clearly enough.