doc: add bitcoind man description

Author: willcl-ark

src/bitcoind.cpp

@@ -136,12 +136,18 @@ static bool ProcessInitCommands(ArgsManager& args)
 {
     // Process help and version before taking care about datadir
     if (HelpRequested(args) || args.IsArgSet("-version")) {
-        std::string strUsage = CLIENT_NAME " version " + FormatFullVersion() + "\n";
+        std::string strUsage = CLIENT_NAME " daemon version " + FormatFullVersion() + "\n";
 
         if (args.IsArgSet("-version")) {
             strUsage += FormatParagraph(LicenseInfo());
         } else {
-            strUsage += "\nUsage:  bitcoind [options]                     Start " CLIENT_NAME "\n"
+            strUsage += "\n"
+                "The " CLIENT_NAME " daemon (bitcoind) is a headless program that connects to the Bitcoin network to validate and relay transactions and blocks, as well as relaying addresses.\n\n"
+                "It provides the backbone of the Bitcoin network and its RPC, REST and ZMQ services can provide various transaction, block and address-related services.\n\n"
+                "There is an optional wallet component which provides transaction services.\n\n"
+                "It can be used in a headless environment or as part of a server setup.\n"
+                "\n"
+                "Usage: bitcoind [options]\n"
                 "\n";
             strUsage += args.GetHelpMessage();
         }