Improve stackforge doc - more info about jobs.

Change-Id: I28cccc53d01813591d2fdccc80a0587b69388c03
Reviewed-on: https://review.openstack.org/25215
Reviewed-by: Khai Do <zaro0508@gmail.com>
Reviewed-by: Jeremy Stanley <fungi@yuggoth.org>
Approved: James E. Blair <corvus@inaugust.com>
Reviewed-by: James E. Blair <corvus@inaugust.com>
Tested-by: Jenkins