workqueue: Fixes for v6.5-rc5

Two commits:

* The recently added cpu_intensive auto detection and warning mechanism was
  spuriously triggered on slow CPUs. While not causing serious issues, it's
  still a nuisance and can cause unintended concurrency management
  behaviors. Relax the threshold on machines with lower BogoMIPS. While
  BogoMIPS is not an accurate measure of performance by most measures, we
  don't have to be accurate and it has rough but strong enough correlation.

* A correction in Kconfig help text.
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