Stabilize CpuClock, make non-exhaustive, rename variants

[bjoernQ]

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Submission Checklist 📝

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Pull Request Details 📖

Description

• mark unstable items in clocks as unstable or doc-hide
• rename enum variants
• make CpuClock non-exhaustive

Testing

CI (failing SPI tests on ESP32 seem to be unrelated)

[MabezDev]

LGTM, thanks!

[jessebraham]

Thanks!