Update comments

Signed-off-by: Haizhi Geng <[email protected]>

server/tso/tso.go

@@ -41,6 +41,9 @@ const (
 
 // TimestampOracle is used to maintain the logic of tso.
 type TimestampOracle struct {
+	// leadership is used to check the current PD server's leadership
+	// to determine whether a tso request could be processed and
+	// it's stored as *election.Leadership
 	leadership atomic.Value
 	// For tso, set after pd becomes leader.
 	ts            unsafe.Pointer

tests/cluster.go

@@ -329,7 +329,8 @@ func (s *TestServer) BootstrapCluster() error {
 	return nil
 }
 
-// WaitLeadership is used to get leader lease.
+// WaitLeadership is used to get instant leadership in order to
+// make a test know the PD leader has been elected as soon as possible.
 // If it exceeds the maximum number of loops, it will return nil.
 func (s *TestServer) WaitLeadership() bool {
 	for i := 0; i < 100; i++ {