Merge #1323: tweak_add: fix API doc for tweak=0

05873bb tweak_add: fix API doc for tweak=0 (Jonas Nick)

Pull request description:

ACKs for top commit:
  real-or-random:
    ACK 05873bb

Tree-SHA512: ef587a680c3355c6328dd61e0f5fcac80ea995f6045b4392fe35f3ee1c04ee1bd941662c120758ad641588670c1f0f53bfb17a802821f54100f1385b8bb7375a

Author: jonasnick

include/secp256k1.h

@@ -734,10 +734,10 @@ SECP256K1_API SECP256K1_WARN_UNUSED_RESULT int secp256k1_ec_pubkey_negate(
  *                  invalid according to secp256k1_ec_seckey_verify, this
  *                  function returns 0. seckey will be set to some unspecified
  *                  value if this function returns 0.
- *  In:    tweak32: pointer to a 32-byte tweak. If the tweak is invalid according to
- *                  secp256k1_ec_seckey_verify, this function returns 0. For
- *                  uniformly random 32-byte arrays the chance of being invalid
- *                  is negligible (around 1 in 2^128).
+ *  In:    tweak32: pointer to a 32-byte tweak, which must be valid according to
+ *                  secp256k1_ec_seckey_verify or 32 zero bytes. For uniformly
+ *                  random 32-byte tweaks, the chance of being invalid is
+ *                  negligible (around 1 in 2^128).
  */
 SECP256K1_API SECP256K1_WARN_UNUSED_RESULT int secp256k1_ec_seckey_tweak_add(
     const secp256k1_context *ctx,
@@ -762,10 +762,10 @@ SECP256K1_API SECP256K1_WARN_UNUSED_RESULT int secp256k1_ec_privkey_tweak_add(
  *  Args:    ctx:   pointer to a context object.
  *  In/Out: pubkey: pointer to a public key object. pubkey will be set to an
  *                  invalid value if this function returns 0.
- *  In:    tweak32: pointer to a 32-byte tweak. If the tweak is invalid according to
- *                  secp256k1_ec_seckey_verify, this function returns 0. For
- *                  uniformly random 32-byte arrays the chance of being invalid
- *                  is negligible (around 1 in 2^128).
+ *  In:    tweak32: pointer to a 32-byte tweak, which must be valid according to
+ *                  secp256k1_ec_seckey_verify or 32 zero bytes. For uniformly
+ *                  random 32-byte tweaks, the chance of being invalid is
+ *                  negligible (around 1 in 2^128).
  */
 SECP256K1_API SECP256K1_WARN_UNUSED_RESULT int secp256k1_ec_pubkey_tweak_add(
     const secp256k1_context *ctx,

include/secp256k1_extrakeys.h

@@ -112,10 +112,10 @@ SECP256K1_API SECP256K1_WARN_UNUSED_RESULT int secp256k1_xonly_pubkey_from_pubke
  *  Out:  output_pubkey: pointer to a public key to store the result. Will be set
  *                       to an invalid value if this function returns 0.
  *  In: internal_pubkey: pointer to an x-only pubkey to apply the tweak to.
- *              tweak32: pointer to a 32-byte tweak. If the tweak is invalid
- *                       according to secp256k1_ec_seckey_verify, this function
- *                       returns 0. For uniformly random 32-byte arrays the
- *                       chance of being invalid is negligible (around 1 in 2^128).
+ *              tweak32: pointer to a 32-byte tweak, which must be valid
+ *                       according to secp256k1_ec_seckey_verify or 32 zero
+ *                       bytes. For uniformly random 32-byte tweaks, the chance of
+ *                       being invalid is negligible (around 1 in 2^128).
  */
 SECP256K1_API SECP256K1_WARN_UNUSED_RESULT int secp256k1_xonly_pubkey_tweak_add(
     const secp256k1_context *ctx,
@@ -229,10 +229,10 @@ SECP256K1_API SECP256K1_WARN_UNUSED_RESULT int secp256k1_keypair_xonly_pub(
  *  Args:       ctx: pointer to a context object.
  *  In/Out: keypair: pointer to a keypair to apply the tweak to. Will be set to
  *                   an invalid value if this function returns 0.
- *  In:     tweak32: pointer to a 32-byte tweak. If the tweak is invalid according
- *                   to secp256k1_ec_seckey_verify, this function returns 0. For
- *                   uniformly random 32-byte arrays the chance of being invalid
- *                   is negligible (around 1 in 2^128).
+ *  In:     tweak32: pointer to a 32-byte tweak, which must be valid according to
+ *                   secp256k1_ec_seckey_verify or 32 zero bytes. For uniformly
+ *                   random 32-byte tweaks, the chance of being invalid is
+ *                   negligible (around 1 in 2^128).
  */
 SECP256K1_API SECP256K1_WARN_UNUSED_RESULT int secp256k1_keypair_xonly_tweak_add(
     const secp256k1_context *ctx,