Make explicit that `@export` is needed in custom `Resource`

**Your Godot version:**

4.1

**Issue description:**

I'm new to custom `Resource`, but I don't think that this is a Godot bug, it is a documentation gap. All variables to be saved from a custom `Resource` must be `@export`. **That is not indicated in documentation** but code examples use it.

It would also be nice to mention that to save modification, you have to use `ResourceSaver`. It only appears in a warning.

**URL to the documentation page (if already existing):**

https://docs.godotengine.org/en/stable/tutorials/scripting/resources.html