Make the or-grouping of `only:variables` explicit in the doc

What does this MR do?

This MR improves clarity on the behaviour of the only:variables config array in gitlab-ci. While other entries in job_control make it obvious that the array is reduced with OR, the variable section does not. This change might help people reading the doc "in anger" while fiddling with a job config.

Does this MR meet the acceptance criteria?

This is purely a doc upgrade, so the whole conformity checklist doesn't seem to be applicable here.