[doc] update Session documentation
A major refactoring of Session/Transaction handling is commit. This is a good
occasion to improves the documention.
# -*- coding: utf-8 -*-
# copyright %(year)s %(author)s, all rights reserved.
# contact %(author-web-site)s -- mailto:%(author-email)s
#
%(long-license)s
"""%(distname)s specific hooks and operations"""