cooldownLimiter began as the wake-only throttle; the OTP-start hardening
reused it via the atomic reserve/release. Its type comment still called it
a per-server wake limiter and justified the per-replica behaviour as
"acceptable because the operator reconcile is idempotent" -- true for wake,
false for OTP, whose every admitted send is a non-idempotent email.
Rewrite the comment to describe the shared per-key limiter and record the
honest KNOWN-LIMITATION: the atomic reserve/release closes the
intra-replica concurrent burst, but the in-memory map throttles per
replica, so cross-replica bounding still needs a shared store. No
behaviour change.
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