Update tree traversal C documentation

[jeromekelleher]

#1862 updates the C code for tree traversals but not the documentation. We should revisit this.

A different structure might be appropriate now also, given we have jupyterbook.

[jeromekelleher]

Probably worth doing this at the same time as #1788

[jeromekelleher]

Can close this when we have API docs for the traversal methods, and have reviewed the C level traversal docs.