What should I tell someone so they can restore my bitcoin from a seed in 20 years?
You probably can get that engraved or something since its a general background information. 

But if you do not have the room or want to stamp that much, you could probably get away with a few key words such as wallet recovery seed, trezor, BIP39, etc. One could search those few terms and find out what it's for. Although, it is a bit of a security risk if a random person finds it.