Improve discoverability of rules:exists: issue

What does this MR do?

The full explanation/backstory for this change proposal is in !130267 (closed).

The tl;dr summary is: There is a fairly simple workaround for the issue mentioned in this troubleshooting item. However, this workaround is generic (not limited to just the SAST jobs, but would also apply to any jobs that users define themselves) and the rules:exists: CI YAML reference section might be the better place for this information.

After some discussion we decided not to document the workaround itself, and focussed on making the existing troubleshooting item more discoverable and tiny bit more specific.

Related issues

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