clean up comment (#2760)

* *: clean up comment
ActivePendingTxn is an interface of context, not Session

* *: Update comment

session.go

@@ -948,7 +948,7 @@ func (s *session) prepareTxnCtx() {
 	}
 }
 
-// ActivePendingTxn implements Session.ActivePendingTxn interface.
+// ActivePendingTxn implements Context.ActivePendingTxn interface.
 func (s *session) ActivePendingTxn() error {
 	if s.txn != nil && s.txn.Valid() {
 		return nil