Clarify order of Descendants, Traverse, and ReverseTraverse

[clbarnes]

I've had a look around and it doesn't seem like "tree order" has any meaning when it comes to graph traversal - rather, it relates to the number of children each node can have. The language in common use for tree traversal seems to be "depth first" and "breadth first", in "pre order", "post order", or "level order".

[saschagrunert]

Hey, this sounds like a good documentation enhancement to me. Do you mind spinning up a PR to fix it? :)

[clbarnes]

Seems like they're all a pre-order depth first search, the only distinction being that descendants yields NodeIds (not &Nodes as the current docs imply); traverse yields NodeEdges containing NodeIds (again, not &Nodes) in depth-first order, addressing the children in insertion order; reverse_traverse addresses children in reverse insertion order. traverse and reverse_traverse visit each node twice (once before its children, once after).