everything.
If they have passed the homecheck I imagine they know the basics, but you'll also want to include most of the rwaf leaflets (you can print them), a list of all the veg/fruit/forage they are fine with and their likes/dislikes, a description of any behaviours, the hays they like and the amount they usually consume so the owner knows what is normal for the bun, and full veterinary history including vaccination card.