tidb: add comment

driver.go

@@ -366,6 +366,10 @@ func newdriverRows(rs rset.Recordset) *driverRows {
 	}
 	r.wg.Add(1)
 	go func() {
+		// We may change the whole implementation later, so here just using WaitGroup
+		// to solve issue https://github.com/pingcap/tidb/issues/57
+		// But if we forget close rows and do commit later, we may still meet this panic
+		// with very little probability.
 		defer r.wg.Done()
 		err := io.EOF
 		if e := r.rs.Do(func(data []interface{}) (bool, error) {