Merge bitcoin-core#664: Remove mention of ec_privkey_export because it doesn't exist

59782c6 Remove mention of ec_privkey_export because it doesn't exist (Jonas Nick)

Pull request description:

  Fixes bitcoin-core#663
  There is `ec_privkey_export_der` but it takes `0` for uncompressed and not `SECP256K1_EC_UNCOMPRESSED` (which is `2`).

ACKs for top commit:
  real-or-random:
    ACK bitcoin-core@59782c6
  apoelstra:
    utACK bitcoin-core@59782c6

Tree-SHA512: 6167581df74264be576f921d04bb8e23e16fa3b823bac4b45299079ceee38d6c74dd14a55b7b976a2cee9bdbd74dd6e3b39c0482808c1b8e65c8c80743f113a2

Author: jonasnick

include/secp256k1.h

@@ -170,7 +170,7 @@ typedef int (*secp256k1_nonce_function)(
 #define SECP256K1_CONTEXT_SIGN (SECP256K1_FLAGS_TYPE_CONTEXT | SECP256K1_FLAGS_BIT_CONTEXT_SIGN)
 #define SECP256K1_CONTEXT_NONE (SECP256K1_FLAGS_TYPE_CONTEXT)
 
-/** Flag to pass to secp256k1_ec_pubkey_serialize and secp256k1_ec_privkey_export. */
+/** Flag to pass to secp256k1_ec_pubkey_serialize. */
 #define SECP256K1_EC_COMPRESSED (SECP256K1_FLAGS_TYPE_COMPRESSION | SECP256K1_FLAGS_BIT_COMPRESSION)
 #define SECP256K1_EC_UNCOMPRESSED (SECP256K1_FLAGS_TYPE_COMPRESSION)