Expand traverse and descendants documentation: Issue #369

[kaesluder]

original traverse documentation is a shortened version of descendants
documentation. Edited documentation comments to clarify the following:

1. traverse and reverse_traverse return an iterator of NodeEdge
   objects. NodeEdge objects mark start (similar to <div>) and end
   (similar to </div>) of each node.

2. descendants returns an iterator of Node objects. This is probably
   more useful if you want to play with the Node contents.

3. added a short "Similar Functions" section to the inline documentation
   to point users in the right direction.

Improves on issue #369

[kivikakk]

Thank you!