add more comments

Signed-off-by: rleungx <rleungx@gmail.com>

server/schedulers/balance_region.go

@@ -58,6 +58,7 @@ const (
 	// balanceRegionRetryLimit is the limit to retry schedule for selected store.
 	balanceRegionRetryLimit = 10
 	// BalanceEmptyRegionThreshold is a threshold which allow balance the empty region if the region number is less than this threshold.
+	// It is mainly used to keep the same as the original whose regions are balanced when initializing a new cluster.
 	balanceEmptyRegionThreshold = 50
 	// BalanceRegionName is balance region scheduler name.
 	BalanceRegionName = "balance-region-scheduler"

server/schedulers/balance_test.go

@@ -962,6 +962,7 @@ func (s *testBalanceRegionSchedulerSuite) TestEmptyRegion(c *C) {
 	operators := sb.Schedule(tc)
 	c.Assert(operators, NotNil)
 
+	// exceed balanceEmptyRegionThreshold
 	for i := uint64(10); i < 60; i++ {
 		tc.PutRegionStores(i, 1, 3, 4)
 	}