doc: document the length requirements of output parameter.

Summary:
It's subtle, since it is actually only touched by hashfp (though
we assert it's non-NULL), but give explicit advice in the default
case.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

This is a backport of libsecp256k1 [[ bitcoin-core/secp256k1#714 | PR714 ]].

Test Plan:
  ninja check-secp256k1

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D5389

src/secp256k1/include/secp256k1_ecdh.h

@@ -22,22 +22,25 @@ typedef int (*secp256k1_ecdh_hash_function)(
  void *data
 );
 
-/** An implementation of SHA256 hash function that applies to compressed public key. */
+/** An implementation of SHA256 hash function that applies to compressed public key.
+ * Populates the output parameter with 32 bytes. */
 SECP256K1_API extern const secp256k1_ecdh_hash_function secp256k1_ecdh_hash_function_sha256;
 
-/** A default ecdh hash function (currently equal to secp256k1_ecdh_hash_function_sha256). */
+/** A default ecdh hash function (currently equal to secp256k1_ecdh_hash_function_sha256).
+ * Populates the output parameter with 32 bytes. */
 SECP256K1_API extern const secp256k1_ecdh_hash_function secp256k1_ecdh_hash_function_default;
 
 /** Compute an EC Diffie-Hellman secret in constant time
  * Returns: 1: exponentiation was successful
  * 0: scalar was invalid (zero or overflow)
  * Args: ctx: pointer to a context object (cannot be NULL)
- * Out: output: pointer to an array to be filled by the function
+ * Out: output: pointer to an array to be filled by hashfp
  * In: pubkey: a pointer to a secp256k1_pubkey containing an
  * initialized public key
  * privkey: a 32-byte scalar with which to multiply the point
  * hashfp: pointer to a hash function. If NULL, secp256k1_ecdh_hash_function_sha256 is used
- * data: Arbitrary data pointer that is passed through
+ * (in which case, 32 bytes will be written to output)
+ * data: Arbitrary data pointer that is passed through to hashfp
  */
 SECP256K1_API SECP256K1_WARN_UNUSED_RESULT int secp256k1_ecdh(
  const secp256k1_context* ctx,