### Godot version

v4.1.1.stable.official [bd6af8e0e]

### System information

Windows 10

### Issue description

As said in the Title Custom Class Documentation a Function cannot return 'Variant' and will instead be shown to return void, even though Built-in Classes are shown to return 'Variant', an example would be 'Array'
![image](https://github.com/godotengine/godot/assets/34311843/fb8ad532-c37a-43c0-a95f-d70af7ea6894)
![image](https://github.com/godotengine/godot/assets/34311843/3cc1e07b-02e1-4f33-bc98-0fa016b4d864)

### Steps to reproduce

```gdscript
extends Node
class_name CustomClass

func VariantFunction() -> Variant:
	return 10
``` 
Now when checking the Generated Documentation it will appear like: `void VariantFunction()`

### Minimal reproduction project

N/A
#include "gdscript_docgen.h"
#include "../gdscript.h"
using GDP = GDScriptParser;
using GDType = GDP::DataType;
static String _get_script_path(const String &p_path) {
	return vformat(R"("%s")", p_path.get_slice("://", 1));
}
static String _get_class_name(const GDP::ClassNode &p_class) {