Clarify error message when invalid -rpcallowip

Also add to HelpMessage() what specifications are valid.

src/init.cpp

@@ -323,7 +323,7 @@ std::string HelpMessage(HelpMessageMode mode)
     strUsage += "  -rpcuser=<user>        " + _("Username for JSON-RPC connections") + "\n";
     strUsage += "  -rpcpassword=<pw>      " + _("Password for JSON-RPC connections") + "\n";
     strUsage += "  -rpcport=<port>        " + _("Listen for JSON-RPC connections on <port> (default: 8332 or testnet: 18332)") + "\n";
-    strUsage += "  -rpcallowip=<ip>       " + _("Allow JSON-RPC connections from specified IP address. This option can be specified multiple times") + "\n";
+    strUsage += "  -rpcallowip=<ip>       " + _("Allow JSON-RPC connections from specified source. Valid for <ip> are a single IP (e.g. 1.2.3.4), a network/netmask (e.g. 1.2.3.4/255.255.255.0) or a network/CIDR (e.g. 1.2.3.4/24). This option can be specified multiple times") + "\n";
     strUsage += "  -rpcthreads=<n>        " + _("Set the number of threads to service RPC calls (default: 4)") + "\n";
 
     strUsage += "\n" + _("RPC SSL options: (see the Bitcoin Wiki for SSL setup instructions)") + "\n";

src/rpcserver.cpp

@@ -512,7 +512,7 @@ void StartRPCThreads()
             if(!subnet.IsValid())
             {
                 uiInterface.ThreadSafeMessageBox(
-                    strprintf("Invalid -rpcallowip subnet specification: %s", strAllow),
+                    strprintf("Invalid -rpcallowip subnet specification: %s. Valid are a single IP (e.g. 1.2.3.4), a network/netmask (e.g. 1.2.3.4/255.255.255.0) or a network/CIDR (e.g. 1.2.3.4/24).", strAllow),
                     "", CClientUIInterface::MSG_ERROR);
                 StartShutdown();
                 return;