TiDB support `help` statement

## Change Request
同样的mysql命令行，mysql数据库help show 可以显示具体的help contents；
而TiDB不能显示，是不是缺失了帮助信息？
mysql> help show
ERROR 8108 (HY000): Unsupported type *ast.HelpStmt

mysql 命令行版本信息如下：
[root@centos76_vm ~]# mysql -uroot -pTidb123! -h 172.16.7.223 -P4000
mysql: [Warning] Using a password on the command line interface can be insecure.
Welcome to the MySQL monitor. Commands end with ; or \g.
Your MySQL connection id is 15
Server version: 5.7.25-TiDB-v5.4.0 TiDB Server (Apache License 2.0) Community Edition, MySQL 5.7 compatible

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Oracle is a registered trademark of Oracle Corporation and/or its
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Type 'help;' or '\h' for help. Type '\c' to clear the current input statement.