doc: document the length requirements of output parameter.
doc: document the length requirements of output parameter.

It's subtle, since it is actually only touched by hashfp (though
we assert it's non-NULL), but give explicit advice in the default
case.

Signed-off-by: Rusty Russell <df9728c9e5104131c08c7adb03af425394842596@rustcorp.com.au>

bitcoin-core/secp256k1,evoskuil/secp256k1,peterdettman/secp256k1,afk11/secp256k1,ElementsProject/secp256k1-zkp,bitcoin/secp256k1,afk11/secp256k1,afk11/secp256k1,evoskuil/secp256k1,bitcoin-core/secp256k1,ElementsProject/secp256k1-zkp,bitcoin/secp256k1,bitcoin/secp256k1,evoskuil/secp256k1,afk11/secp256k1,bitcoin/secp256k1,afk11/secp256k1,evoskuil/secp256k1,evoskuil/secp256k1,bitcoin/secp256k1,peterdettman/secp256k1