Clarify a bit more when to use Broken Pipeline label

Why is this change being made?

Clarify when to use ~"Broken Pipeline". Some pipeline failures might require follow-up issues, but this does not mean that the follow-up issue will be used to fix a currently broken pipeline. It might be that the pipeline was already fixed by any other means and the follow-up issue is to make the pipeline more robust.

The previous wording gave me the feeling that any follow-up issue related to a pipeline that was once broken, would have to have the Broken Pipeline label. But my understanding now is that this is only needed for follow-up issues if the pipeline actually still is broken.

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