The documentation you need is similar to what it would have been required for a loan from an unrelated third party.

You should not rely on what the parties call it but review the document for intent, whether it was a bona fide loan that was evidenced by documentation, whether the terms are comparable to those in the market, enforceable, and were actually carried out, whether interest is chargeable at AFR at a minimum, and how the related transactions were recorded on the books.   You should be able to trace back from the documentation and the books what the difference is.