docs: correct pubkey param descriptions for secp256k1_keypair_{xonly_,}pub

From an API perspective, the functions `secp256k1_keypair_pub` and
`secp256k1_keypair_xonly_pub` always succeed (i.e. return the value 1),
so the other cases in the `pubkey` parameter descriptions never happen
and can hence be removed.

Note that the "1 always" return value description was previously done in
commit b8f8b99 (PR bitcoin#1089), which also
explains why invalid inputs for the affected functions are in practice
only possible in violation of the type system.

Author: theStack

include/secp256k1_extrakeys.h

@@ -185,9 +185,8 @@ SECP256K1_API SECP256K1_WARN_UNUSED_RESULT int secp256k1_keypair_sec(
 /** Get the public key from a keypair.
  *
  *  Returns: 1 always.
- *  Args:    ctx: pointer to a context object.
- *  Out: pubkey: pointer to a pubkey object. If 1 is returned, it is set to
- *               the keypair public key. If not, it's set to an invalid value.
+ *  Args:   ctx: pointer to a context object.
+ *  Out: pubkey: pointer to a pubkey object, set to the keypair public key.
  *  In: keypair: pointer to a keypair.
  */
 SECP256K1_API SECP256K1_WARN_UNUSED_RESULT int secp256k1_keypair_pub(
@@ -203,9 +202,8 @@ SECP256K1_API SECP256K1_WARN_UNUSED_RESULT int secp256k1_keypair_pub(
  *
  *  Returns: 1 always.
  *  Args:   ctx: pointer to a context object.
- *  Out: pubkey: pointer to an xonly_pubkey object. If 1 is returned, it is set
- *               to the keypair public key after converting it to an
- *               xonly_pubkey. If not, it's set to an invalid value.
+ *  Out: pubkey: pointer to an xonly_pubkey object, set to the keypair
+ *               public key after converting it to an xonly_pubkey.
  *    pk_parity: Ignored if NULL. Otherwise, pointer to an integer that will be set to the
  *               pk_parity argument of secp256k1_xonly_pubkey_from_pubkey.
  *  In: keypair: pointer to a keypair.