update cli help

make `gunicorn -h` identical with source code.
Well, I'm also OK with something like "Turn on debugging in the server. (deprecated: no-op as of v18.0)". For example:

```
  --debug               Turn on debugging in the server. (deprecated: no-op as
                        of v18.0) [False]
```

This description is also using to generate the settings documentation at http://docs.gunicorn.org/en/latest/settings.html#debug, so it would be good to keep the actual description of the option in the first line.