scalar: extend docstring of secp256k1_scalar_set_b32
scalar: extend docstring of secp256k1_scalar_set_b32

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