希望支持类似MySQL的\help语法

在MySQL里，类似下面的用法
```
mysql> \help select
Name: 'SELECT'
Description:
Syntax:
SELECT
    [ALL | DISTINCT | DISTINCTROW ]
    [HIGH_PRIORITY]
...
SELECT can also be used to retrieve rows computed without reference to
any table.

URL: https://dev.mysql.com/doc/refman/8.0/en/select.html
```

建议在obclient中也能支持。