[book] fix sphinx documentation generation (closes #2991997) changeset 17994bf95d6a ([doc] update Session documentation) added cubicweb.server.session.Session autodoc to the book. This caused errors from sphinx due to a section title not being allowed in the class's docstring.

# -*- 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 repository side views, usually for notification"""