rpc: sendrawtransaction unconditionality/privacy note

In sendrawtransaction RPCHelpMan, mention unconditionality and privacy
as per http://www.erisian.com.au/bitcoin-core-dev/log-2019-07-25.html#l-522

Thank you to MarcoFalke and laanwj for their review and suggestions.

Author: jonatack

src/rpc/rawtransaction.cpp

@@ -760,7 +760,10 @@ static UniValue signrawtransactionwithkey(const JSONRPCRequest& request)
 static UniValue sendrawtransaction(const JSONRPCRequest& request)
 {
     RPCHelpMan{"sendrawtransaction",
-                "\nSubmits raw transaction (serialized, hex-encoded) to local node and network.\n"
+                "\nSubmit a raw transaction (serialized, hex-encoded) to local node and network.\n"
+                "\nNote that the transaction will be sent unconditionally to all peers, so using this\n"
+                "for manual rebroadcast may degrade privacy by leaking the transaction's origin, as\n"
+                "nodes will normally not rebroadcast non-wallet transactions already in their mempool.\n"
                 "\nAlso see createrawtransaction and signrawtransactionwithkey calls.\n",
                 {
                     {"hexstring", RPCArg::Type::STR_HEX, RPCArg::Optional::NO, "The hex string of the raw transaction"},