Merge bitcoin-core/secp256k1#1296: docs: complete interface descripti…

…on for `secp256k1_schnorrsig_sign_custom`

149c41c docs: complete interface description for `secp256k1_schnorrsig_sign_custom` (Sebastian Falbesoner)

Pull request description:

ACKs for top commit:
  real-or-random:
    utACK 149c41c
  jonasnick:
    ACK 149c41c

Tree-SHA512: ee677ed6b474b547066ce149688edab7ba6d2572acfbc0989256a669341fff4cf2e17b451cd3fc6fff3944a896647f0f5c1411056678505fa85ba71e8cfe6229

include/secp256k1_schnorrsig.h

@@ -144,9 +144,13 @@ SECP256K1_API int secp256k1_schnorrsig_sign(
  *  Creates the same signatures as schnorrsig_sign if msglen is 32 and the
  *  extraparams.ndata is the same as aux_rand32.
  *
+ *  Returns 1 on success, 0 on failure.
+ *  Args:   ctx: pointer to a context object (not secp256k1_context_static).
+ *  Out:  sig64: pointer to a 64-byte array to store the serialized signature.
  *  In:     msg: the message being signed. Can only be NULL if msglen is 0.
- *       msglen: length of the message
- *  extraparams: pointer to a extraparams object (can be NULL)
+ *       msglen: length of the message.
+ *      keypair: pointer to an initialized keypair.
+ *  extraparams: pointer to an extraparams object (can be NULL).
  */
 SECP256K1_API int secp256k1_schnorrsig_sign_custom(
     const secp256k1_context *ctx,