doc: Mention blocksonly in reduce-traffic.md, unhide option

Github-Pull: #15990
Rebased-From: fa8ced32a6

doc/reduce-traffic.md

@@ -35,3 +35,16 @@ blocks and transactions to fewer nodes.
 Reducing the maximum connected nodes to a minimum could be desirable if traffic
 limits are tiny. Keep in mind that bitcoin's trustless model works best if you are
 connected to a handful of nodes.
+
+## 4. Turn off transaction relay (`-blocksonly`)
+
+Forwarding transactions to peers increases the P2P traffic. To only sync blocks
+with other peers, you can disable transaction relay.
+
+Be reminded of the effects of this setting.
+
+- Fee estimation will no longer work.
+- Not relaying other's transactions could hurt your privacy if used while a
+  wallet is loaded or if you use the node to broadcast transactions.
+- It makes block propagation slower because compact block relay can only be
+  used when transaction relay is enabled.

src/init.cpp

@@ -376,7 +376,7 @@ void SetupServerArgs()
     gArgs.AddArg("-blocksdir=<dir>", "Specify blocks directory (default: <datadir>/blocks)", false, OptionsCategory::OPTIONS);
     gArgs.AddArg("-blocknotify=<cmd>", "Execute command when the best block changes (%s in cmd is replaced by block hash)", false, OptionsCategory::OPTIONS);
     gArgs.AddArg("-blockreconstructionextratxn=<n>", strprintf("Extra transactions to keep in memory for compact block reconstructions (default: %u)", DEFAULT_BLOCK_RECONSTRUCTION_EXTRA_TXN), false, OptionsCategory::OPTIONS);
-    gArgs.AddArg("-blocksonly", strprintf("Whether to operate in a blocks only mode (default: %u)", DEFAULT_BLOCKSONLY), true, OptionsCategory::OPTIONS);
+    gArgs.AddArg("-blocksonly", strprintf("Whether to reject transactions from network peers. Transactions from the wallet or RPC are not affected. (default: %u)", DEFAULT_BLOCKSONLY), false, OptionsCategory::OPTIONS);
     gArgs.AddArg("-conf=<file>", strprintf("Specify configuration file. Relative paths will be prefixed by datadir location. (default: %s)", BITCOIN_CONF_FILENAME), false, OptionsCategory::OPTIONS);
     gArgs.AddArg("-datadir=<dir>", "Specify data directory", false, OptionsCategory::OPTIONS);
     gArgs.AddArg("-dbbatchsize", strprintf("Maximum database write batch size in bytes (default: %u)", nDefaultDbBatchSize), true, OptionsCategory::OPTIONS);