Clarify -define help.

src/main.cpp

@@ -2039,8 +2039,8 @@ void print_show_help(String const arg0, String const &command) {
 		print_usage_line(3, "import \"shared:foo\"");
 		print_usage_line(0, "");
 
-		print_usage_line(1, "-define:<name>=<expression>");
-		print_usage_line(2, "Defines a global constant with a value");
+		print_usage_line(1, "-define:<name>=<value>");
+		print_usage_line(2, "Defines a scalar boolean, integer or string as global constant");
 		print_usage_line(2, "Example: -define:SPAM=123");
 		print_usage_line(2, "To use:  #config(SPAM, default_value)");
 		print_usage_line(0, "");