document test variables _START_AFTER_JOBS and similar
In test suites we should use START_AFTER_TEST and similar as documented in http://open.qa/docs/#_spawning_multiple_jobs_based_on_templates_isos_post and http://open.qa/docs/#_job_dependencies . However there are also variants like
_START_AFTER_JOBS as used in
openqa-clone-job. These variables have the benefit that they can be added to individual jobs so not relying on
isos post based on media as such but can still express test dependencies. They should be documented and tested accordingly to be more accessible to users.