[doc] Book entry for the undo functionnality.

[toward py3k] rewrite has_key() (part of #2711624) The `has_key` method is not in Python3K but dict supports __contains__ check for ages.

[toward py3k] rewrite dict.keys() and dict.values() (part of #2711624) Python 3K will only expose iterators. http://docs.python.org/3.0/whatsnew/3.0.html#views-and-iterators-instead-of-lists