Update error message about --datadir

Noone remembers these days that at some point ZNC supported using the
same config directory, but different znc.conf. So now the old message is
just confusing.

However, nowadays many people confuse "/znc foo" in IRC client with
"znc foo" in shell.

src/main.cpp

@@ -248,8 +248,9 @@ int main(int argc, char** argv) {
 	}
 
 	if (optind < argc) {
-		CUtils::PrintError("Specifying a config file as an argument isn't supported anymore.");
-		CUtils::PrintError("Use --datadir instead.");
+		CUtils::PrintError("Unrecognized command line arguments.");
+		CUtils::PrintError("Did you mean to run `/znc " + CString(argv[optind]) + "' in IRC client instead?");
+		CUtils::PrintError("Hint: `/znc " + CString(argv[optind]) + "' is an alias for `/msg *status " + CString(argv[optind]) + "'");
 		return 1;
 	}