Clarify docs that `PayPalNativeShippingDelegate` is sometimes required

[scannillo]

Reason for changes

• This GH issue raised a question about when a merchant needs to implement the shipping callback. Turns out, it is not entirely optional. There is a case when a merchant must set it to avoid an infinite spinner on their checkout button.
• Special thanks to @laurentbourg for highlighting this area of improvement 👏

Summary of changes

• Clarify that merchants using GET_FROM_FILE orders must implement PayPalNativeShippingDelegate

Checklist

• ~Added a changelog entry~

Authors

@scannillo