doc: Update bitcoin-tx replaceable documentation

Author: kashifs

src/bitcoin-tx.cpp

@@ -66,7 +66,9 @@ static void SetupBitcoinTxArgs(ArgsManager &argsman)
     argsman.AddArg("outscript=VALUE:SCRIPT[:FLAGS]", "Add raw script output to TX. "
         "Optionally add the \"W\" flag to produce a pay-to-witness-script-hash output. "
         "Optionally add the \"S\" flag to wrap the output in a pay-to-script-hash.", ArgsManager::ALLOW_ANY, OptionsCategory::COMMANDS);
-    argsman.AddArg("replaceable(=N)", "Set RBF opt-in sequence number for input N (if not provided, opt-in all available inputs)", ArgsManager::ALLOW_ANY, OptionsCategory::COMMANDS);
+    argsman.AddArg("replaceable(=N)", "Sets Replace-By-Fee (RBF) opt-in sequence number for input N. "
+        "If N is not provided, the command attempts to opt-in all available inputs for RBF. "
+        "If the transaction has no inputs, this option is ignored.", ArgsManager::ALLOW_ANY, OptionsCategory::COMMANDS);
     argsman.AddArg("sign=SIGHASH-FLAGS", "Add zero or more signatures to transaction. "
         "This command requires JSON registers:"
         "prevtxs=JSON object, "