Fix Sphinx 2.x warnings

This fixes the following Sphinx 2.x warnings in swh-web:

/home/antoine/swh/swh-environment/swh-web/swh/web/browse/utils.py:docstring of swh.web.browse.utils.get_snapshot_context:: WARNING: plusieurs cibles trouvées pour le renvoi 'NotFoundExc' : swh.vault.backend.NotFoundExc, swh.web.common.exc.NotFoundExc
/home/antoine/swh/swh-environment/swh-web/swh/web/common/origin_save.py:docstring of swh.web.common.origin_save.get_save_origin_requests:: WARNING: plusieurs cibles trouvées pour le renvoi 'NotFoundExc' : swh.vault.backend.NotFoundExc, swh.web.common.exc.NotFoundExc
/home/antoine/swh/swh-environment/swh-web/swh/web/common/origin_visits.py:docstring of swh.web.common.origin_visits.get_origin_visits:: WARNING: plusieurs cibles trouvées pour le renvoi 'NotFoundExc' : swh.vault.backend.NotFoundExc, swh.web.common.exc.NotFoundExc
/home/antoine/swh/swh-environment/swh-web/swh/web/common/service.py:docstring of swh.web.common.service.lookup_revision_log:: WARNING: plusieurs cibles trouvées pour le renvoi 'NotFoundExc' : swh.vault.backend.NotFoundExc, swh.web.common.exc.NotFoundExc
/home/antoine/swh/swh-environment/swh-web/swh/web/common/service.py:docstring of swh.web.common.service.lookup_revision_log_by:: WARNING: plusieurs cibles trouvées pour le renvoi 'NotFoundExc' : swh.vault.backend.NotFoundExc, swh.web.common.exc.NotFoundExc
/home/antoine/swh/swh-environment/swh-web/swh/web/common/service.py:docstring of swh.web.common.service.lookup_object:: WARNING: plusieurs cibles trouvées pour le renvoi 'NotFoundExc' : swh.vault.backend.NotFoundExc, swh.web.common.exc.NotFoundExc

This raises the question that maybe we should centralize common used exceptions in swh-core but first let's simply fix the documentation build on Jenkins.

Related to swh-docs#2264 (closed)

---

Migrated from D2618 (view on Phabricator)