Fixed function documentation to new wp-cli standards

I got this failure when running:
```
$ wp migrate to /srv/http/test.dev http://test.dev/ test.sql   
Warning: The `wp migrate to` command has an invalid synopsis part: output_folder
Warning: The `wp migrate to` command has an invalid synopsis part: output_host
Warning: The `wp migrate to` command has an invalid synopsis part: filename
Error: Too many positional arguments: /srv/http/test.dev http://test.dev/ test.sql
```
Now it works