improve ECDH header-doc

- use pubkey instead of point
- use privkey instead of scalar

include/secp256k1_ecdh.h

@@ -13,14 +13,15 @@ extern "C" {
  *  Args:    ctx:        pointer to a context object (cannot be NULL)
  *  Out:     result:     a 32-byte array which will be populated by an ECDH
  *                       secret computed from the point and scalar
- *  In:      point:    pointer to a public point
- *           scalar:   a 32-byte scalar with which to multiply the point
+ *  In:      pubkey:     a pointer to a secp256k1_pubkey containing an
+ *                       initialized public key
+ *           privkey:    a 32-byte scalar with which to multiply the point
  */
 SECP256K1_API SECP256K1_WARN_UNUSED_RESULT int secp256k1_ecdh(
   const secp256k1_context* ctx,
   unsigned char *result,
-  const secp256k1_pubkey *point,
-  const unsigned char *scalar
+  const secp256k1_pubkey *pubkey,
+  const unsigned char *privkey
 ) SECP256K1_ARG_NONNULL(1) SECP256K1_ARG_NONNULL(2) SECP256K1_ARG_NONNULL(3) SECP256K1_ARG_NONNULL(4);
 
 # ifdef __cplusplus