Warn about manually editing files created by the scenario editor (#5001)

This comment will be added at the start of any .cfg file written by the
scenario editor. Given that the only target audience is authors using text
editors, it's not translatable and uses plain ascii characters.

src/editor/map/map_context.cpp

@@ -589,6 +589,14 @@ bool map_context::save_scenario()
 
 	try {
 		std::stringstream wml_stream;
+		wml_stream
+			<< "# This file was generated using the scenario editor.\n"
+			<< "#\n"
+			<< "# If you edit this file by hand, then you shouldn't use the\n"
+			<< "# scenario editor on it afterwards. The editor completely\n"
+			<< "# rewrites the file when it saves it, which will lose any WML\n"
+			<< "# that the editor doesn't support.\n"
+			<< "\n";
 		{
 			config_writer out(wml_stream, false);
 			out.write(to_config());