kvserver: improve a comment in TransferLeaseTarget

Release justification: comments only

Release note: None

pkg/kv/kvserver/allocator.go

@@ -1450,8 +1450,12 @@ func (a *Allocator) TransferLeaseTarget(
 
  leaseholderReplQPS, _ := stats.avgQPS()
  currentDelta := getQPSDelta(storeQPSMap, existing)
+ // NB: If the `bestOption` ends up being the current leaseholder, then the
+ // check below will fail since we know that `leaseholderReplQPS` must be
+ // greater than 0 (the `StoreRebalancer` only bothers rebalancing leases /
+ // replicas for ranges serving at least some traffic).
  bestOption := getCandidateWithMinQPS(storeQPSMap, existing)
- if bestOption != (roachpb.ReplicaDescriptor{}) &&
+ if bestOption != (roachpb.ReplicaDescriptor{}) && leaseholderReplQPS > 0 &&
  // It is always beneficial to transfer the lease to the coldest candidate
  // if the range's own qps is smaller than the difference between the
  // leaseholder store and the candidate store. This will always drive down