Add [EXPERIMENTAL] warning to -user-separate-modules help message

src/main.cpp

@@ -1724,6 +1724,7 @@ void print_show_help(String const arg0, String const &command) {
 		print_usage_line(0, "");
 
 		print_usage_line(1, "-use-separate-modules");
+		print_usage_line(1, "[EXPERIMENTAL]");
 		print_usage_line(2, "The backend generates multiple build units which are then linked together");
 		print_usage_line(2, "Normally, a single build unit is generated for a standard project");
 		print_usage_line(0, "");