For the sake of completeness, add the missing descriptions for the return value and parameters (`ctx`, `sig64`, `keypair`), in the same wording/style as for the function `secp256k1_schnorrsig_sign32`.
For the sake of completeness, add the missing descriptions for the return value and parameters (`ctx`, `sig64`, `keypair`), in the same wording/style as for the function `secp256k1_schnorrsig_sign32`.