Add doc for jobs voting parameter

Change-Id: I14e489a98bbbf245ced2726d0bedc32accc7ca42
Reviewed-on: https://review.openstack.org/32665
Reviewed-by: Clark Boylan <clark.boylan@gmail.com>
Approved: Jeremy Stanley <fungi@yuggoth.org>
Reviewed-by: Jeremy Stanley <fungi@yuggoth.org>
Tested-by: Jenkins