Correct Redo coverage line item values in custom returns integration docs

[Steven Mitchell (smitch5777)]

• Fix _redo_type example value: return -> returns
• Document that _redo_type takes precedence over tags, and that priceSet on the Redo line item drives billing
• Document __final_sale_coverage for final sale returns coverage
• Add a Package Protection Plus section covering shopper-paid vs merchant-paid/all-orders-covered
• Add a Redo coverage line item to the example request

[William Schaefermeyer (WillsWebsites)]

Can we remove all the stuff from this guide that is trying to explain about the requests and schemas and auth and just reference the API endpoint for it. this guide can get a lot more lean and just be truly a guide if we do that