Document module register_(module_name)_types.h new functions

when creating a new module register_xxx_types.h file is needed with functions:
```
void register_xxx_types();
void unregister_xxx_types();
```
these functions were changed to:
```
void initialize_xxx_module(ModuleInitializationLevel p_level);
void uninitialize_xxx_module(ModuleInitializationLevel p_level);
```
but are yet to be added to the docs.
I tried to create a custom module by following [the docs](https://docs.godotengine.org/en/latest/development/cpp/custom_modules_in_cpp.html), but apparently these functions are outdated