Document traversal orders

[jeromekelleher]

The Tree.nodes iterator doesn't provide any description of what the various traversal orders actually are. Probably we should have a description list within the method to describe each one, possibly with links out to Wikipedia or somewhere.

[brianzhang01]

Where specifically would the documentation be added in order to be viewable by a user? If it's straightforward it could be added to #411, though I'm already behind on getting the PR out.

[jeromekelleher]

I think it can go directly into the docstring for the nodes function, so that the list will end up in here

Thanks @brianzhang01!

[brianzhang01]

Cool, I'll add it to my PR. Feel free to assign me.