Clarify details about .pre and .post only pipelines

What does this MR do?

As mentioned in the related issue, some people get confused when a pipeline fails silently (does not run), and it's because all the jobs are in the .pre or .post stages. Since that issue was created, we already added some warning notes in the .pre and .post reference docs, but we should clarify those notes. We should also put it in stages, as requested in the issue.

Related issues

Related to #219416 (closed)

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