Clarify documentation of tweak functions.
Clarify documentation of tweak functions.

In particular, mention that the functions return 0 if seckey or tweak are
invalid (as opposed to saying "should" or "must" be valid).

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