The default value for tidb_txn_mode is different with doc desc

[Defined2014]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

SELECT * FROM information_schema.variables_info WHERE variable_name = 'tidb_txn_mode';

2. What did you expect to see? (Required)

mysql> SELECT * FROM information_schema.variables_info WHERE variable_name = 'tidb_txn_mode';
+---------------+----------------+---------------+---------------+-----------+-----------+------------------------+---------+
| VARIABLE_NAME | VARIABLE_SCOPE | DEFAULT_VALUE | CURRENT_VALUE | MIN_VALUE | MAX_VALUE | POSSIBLE_VALUES        | IS_NOOP |
+---------------+----------------+---------------+---------------+-----------+-----------+------------------------+---------+
| tidb_txn_mode | SESSION,GLOBAL | pessimistic   | pessimistic   |      NULL |      NULL | pessimistic,optimistic | NO      |
+---------------+----------------+---------------+---------------+-----------+-----------+------------------------+---------+
1 row in set (0.01 sec)

3. What did you see instead (Required)

mysql> SELECT * FROM information_schema.variables_info WHERE variable_name = 'tidb_txn_mode';
+---------------+----------------+---------------+---------------+-----------+-----------+------------------------+---------+
| VARIABLE_NAME | VARIABLE_SCOPE | DEFAULT_VALUE | CURRENT_VALUE | MIN_VALUE | MAX_VALUE | POSSIBLE_VALUES        | IS_NOOP |
+---------------+----------------+---------------+---------------+-----------+-----------+------------------------+---------+
| tidb_txn_mode | SESSION,GLOBAL |               | pessimistic   |      NULL |      NULL | pessimistic,optimistic | NO      |
+---------------+----------------+---------------+---------------+-----------+-----------+------------------------+---------+
1 row in set (0.01 sec)

4. What is your TiDB version? (Required)

ref doc link https://docs.pingcap.com/tidb/stable/system-variables#tidb_txn_mode

[Defined2014]

The biggest problem is that when we do set tidb_txn_mode=default it becomes optimistic instead of pessimistic.