Docs and specs for traverse_and_update are wrong

[josevalim]

The docs say that it will receive an html_tag but the callback is also called for doctype/comment/pi nodes. I am not sure if you want to fix the specs/docs or the code. :)

[philss]

@josevalim thank you for catching it! 💜
I think the problem is in the docs, because the callback function needs to handle nodes for doctype/comment/pi in order to be able to change it. So we need to update the docs instead of removing the "feature".

[josevalim]

Cool, PR sent here #289.