Fix explain help text

This is too wide (85 characters). It is also not quite correct due to the comma. We can either remove it or do it like the manual:

```
EXPLAIN [ ( option [, ...] ) ] <query>
    options: FORMAT { TEXT | GRAPHVIZ }
             TYPE { LOGICAL | DISTRIBUTED }
```

I'm not sure we need this level of help in the list of commands. The MySQL CLI lets you type "HELP EXPLAIN" and it shows a text version of the manual. It actually has the full manual (they might be real SQL executed server side).