Clarify CPU frequency stability guidance

[Haihan-Jiang]

Summary:

• soften the CPU governor guidance from a guarantee to a usual variance reduction
• call out that boost, SMT/Hyperthreading, firmware, and kernel behavior can still affect observed frequency
• suggest monitoring per-CPU frequency during sensitive benchmark runs

Fixes #1686.

Test:

• git diff --check

[Haihan-Jiang]

Pushed 5e5d81a to remove the duplicated sentence here; the lower variance section now remains the canonical place for that guidance.