Add request to report missing exposed GDExtension funcationality

* adds note to the "What is GDExtension" docs to call for issue creation when people can't do something in GDExtension that is possible with modules

tutorials/scripting/gdextension/what_is_gdextension.rst

@@ -22,6 +22,11 @@ run C or C++ code in a Godot project.
 They also both allow you to integrate third-party libraries into Godot. The one
 you should choose depends on your needs.
 
+.. note:: If you notice that specific systems are not accessible via GDExtension
+          but are via custom modules feel free to open an issue on the
+          `godot-cpp repository <https://github.com/godotengine/godot-cpp>`__ to discuss
+          implementation options to expose the missing functionality.
+
 Advantages of GDExtension
 ^^^^^^^^^^^^^^^^^^^^^^^^^