[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.

\.svn
^build$
^dist$
\.pyc$
\.pyo$
\.bak$
\.old$
\~$
\#.*?\#$
\.swp$
^doc/book/en/apidoc$
\.old$
syntax: regexp
.*/data/database/.*\.sqlite
.*/data/database/.*\.config
.*/data/database/tmpdb.*
.*/data/ldapdb/.*
^doc/html/
^doc/doctrees/
^doc/book/en/devweb/js_api/