Documentation: Add a warning about rules:changes with no PushEvent

What does this MR do?

This MR adds a warning about the behavior of rules:changes in the context of pipelines with no PushEvent, and also details how those pipelines can be selected for and against in CI configuration by using CI_PIPELINE_SOURCE. This has been confusing for quite awhile, but I was recently pointed to a specific section of the documentation that seemed appropriate for the warning. (Thanks @hartzell!)

There's much more about this in the only:changes section, but as rules: is the prescribed configuration method going forward, I think it makes sense to add a concise warning here.

Related issues

#25855 (closed)

Author's checklist (required)

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When applicable:

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Review checklist

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