As of now, the "description" field on idea custom fields is for internal setup reference, and does not present as descriptive help text on an idea portal showing the field.
It would helpful (for any custom field type) to have an option to provide descriptive help text on the field that is customer-facing on the portal.
Thanks for the idea!
On the portal's 'new idea' form custom fields will display the field's 'description' as placeholder text to help guide users. This currently works for all field types except for a 'predefined choice list'. We will keep this idea open to track customer need and prioritize accordingly.