doc: add missing "testnet4" network string in RPC/init help texts

The following bitcoind parameters / RPC calls missed the "testnet4"
network string:
    - `-chain=` parameter
    - `getblockchaininfo` RPC, "chain" result
    - `getmininginfo` RPC, "chain" result

Author: theStack

src/chainparamsbase.cpp

@@ -13,7 +13,7 @@
 
 void SetupChainParamsBaseOptions(ArgsManager& argsman)
 {
-    argsman.AddArg("-chain=<chain>", "Use the chain <chain> (default: main). Allowed values: main, test, signet, regtest", ArgsManager::ALLOW_ANY, OptionsCategory::CHAINPARAMS);
+    argsman.AddArg("-chain=<chain>", "Use the chain <chain> (default: main). Allowed values: main, test, testnet4, signet, regtest", ArgsManager::ALLOW_ANY, OptionsCategory::CHAINPARAMS);
     argsman.AddArg("-regtest", "Enter regression test mode, which uses a special chain in which blocks can be solved instantly. "
                  "This is intended for regression testing tools and app development. Equivalent to -chain=regtest.", ArgsManager::ALLOW_ANY | ArgsManager::DEBUG_ONLY, OptionsCategory::CHAINPARAMS);
     argsman.AddArg("-testactivationheight=name@height.", "Set the activation height of 'name' (segwit, bip34, dersig, cltv, csv). (regtest-only)", ArgsManager::ALLOW_ANY | ArgsManager::DEBUG_ONLY, OptionsCategory::DEBUG_TEST);

src/rpc/blockchain.cpp

@@ -1273,7 +1273,7 @@ RPCHelpMan getblockchaininfo()
         RPCResult{
             RPCResult::Type::OBJ, "", "",
             {
-                {RPCResult::Type::STR, "chain", "current network name (main, test, signet, regtest)"},
+                {RPCResult::Type::STR, "chain", "current network name (main, test, testnet4, signet, regtest)"},
                 {RPCResult::Type::NUM, "blocks", "the height of the most-work fully-validated chain. The genesis block has height 0"},
                 {RPCResult::Type::NUM, "headers", "the current number of headers we have validated"},
                 {RPCResult::Type::STR, "bestblockhash", "the hash of the currently best block"},

src/rpc/mining.cpp

@@ -422,7 +422,7 @@ static RPCHelpMan getmininginfo()
                         {RPCResult::Type::NUM, "difficulty", "The current difficulty"},
                         {RPCResult::Type::NUM, "networkhashps", "The network hashes per second"},
                         {RPCResult::Type::NUM, "pooledtx", "The size of the mempool"},
-                        {RPCResult::Type::STR, "chain", "current network name (main, test, signet, regtest)"},
+                        {RPCResult::Type::STR, "chain", "current network name (main, test, testnet4, signet, regtest)"},
                         (IsDeprecatedRPCEnabled("warnings") ?
                             RPCResult{RPCResult::Type::STR, "warnings", "any network and blockchain warnings (DEPRECATED)"} :
                             RPCResult{RPCResult::Type::ARR, "warnings", "any network and blockchain warnings (run with `-deprecatedrpc=warnings` to return the latest warning as a single string)",