chore: fix stale doc comments

[simone-stacks]

Four mechanically-falsified comment bugs: stale Err(NotInPreparePhase) claim on get_nakamoto_reward_cycle_info, "Panics if" docs on increase_lock_v2/v3 that actually return Err, copy-paste V3 in the increase_lock_v4 panic doc, and missing // SAFETY: on two DoubleSha256 mem::transmute blocks.

Doc-only. No behaviour change. Result of a first smoke-test of the quality-agent.

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