Clarify that redeemScript is often optional

Author: zw

src/rpcrawtransaction.cpp

@@ -523,7 +523,7 @@ Value signrawtransaction(const Array& params, bool fHelp)
             "         \"txid\":\"id\",             (string, required) The transaction id\n"
             "         \"vout\":n,                  (numeric, required) The output number\n"
             "         \"scriptPubKey\": \"hex\",   (string, required) script key\n"
-            "         \"redeemScript\": \"hex\"    (string, required) redeem script\n"
+            "         \"redeemScript\": \"hex\"    (string, required for P2SH) redeem script\n"
             "       }\n"
             "       ,...\n"
             "    ]\n"