redbot-setup's `--overwrite-existing-instance` and `--debug` flags are missing help description

[Jackenmen]

In order to fix this, update the decorators for those options in redbot\setup.py. The description of these can mostly be copied from redbot\cli.py, some alterations need to be made for the description of --overwrite-existing-instance to fit how it's used in redbot-setup.

The change can be tested by running redbot-setup --help or python -m redbot.setup --help.