[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.
addressbook
basket
blog
book
calendar
comment
company
email
file
folder
i18ncontent
keyword
link
mailinglist
person
tag
timeseries
vcsfile