Improve help and default values printing

[tasooshi]

For example:

• (default: None) (don't print in that case)
• -d, --debug Enable debug mode (verbose output) (default: 20) (log level as a meaningful string like INFO)
• (default: set()) (do not print)
• -dR, --disable-retry Disable retry for failed connections (default: True) (misleading)