Improve the description of -l option

If we add logging option -l DEBUG to the monai-deploy package command, it would affect only for packager module (it would show the messages during executing docker build command), not app itself.

However, it is possible that the user may think that specifying -l DEBUG would make App's default logging level to DEBUG when it's not.

It is good to improve the description of -l option.

Since -l option is from main CLI parser(monai/deploy/cli/main.py) that packager parser(monai/deploy/packager/package_command.py) cannot control, we may need to clarify it in main CLI parser's description of -l option or in the documentation.