Document edge case where job artifacts in blocked pipelines are always marked as latest

Spin off from #266958 (comment 1226877383)

pipeline status is typically blocked due to a manual deployment job not having been run, and all jobs show as having the latest artifacts. This leads to an unwanted accumulation of artifact storage.

"blocked" pipeline is either in manual or scheduled state. These pipeline states don't trigger an update on the ci_ref state.

This issue is to document this edge case in https://docs.gitlab.com/ee/ci/pipelines/job_artifacts.html#keep-artifacts-from-most-recent-successful-jobs