eth/catalyst: add comment about spin loop

eth/catalyst/simulated_beacon_api.go

@@ -61,6 +61,11 @@ func (a *simulatedBeaconAPI) loop() {
 		for _ = range doCommit {
 			a.sim.Commit()
 			a.sim.eth.TxPool().Sync()
+
+			// It's worth noting that in case a tx ends up in the pool listed as
+			// "executable", but for whatever reason the miner does not include it in
+			// a block -- maybe the miner is enforcing a higher tip than the pool --
+			// this code will spinloop.
 			for {
 				if executable, _ := a.sim.eth.TxPool().Stats(); executable == 0 {
 					break