### 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
if (m_func->return_type) {
					mi.return_val = _property_info_from_datatype(m_func->return_type->get_datatype());
				}
				for (const GDScriptParser::ParameterNode *p : m_func->parameters) {
					PropertyInfo pi = _property_info_from_datatype(p->get_datatype());
					pi.name = p->identifier->name;
					mi.arguments.push_back(pi);
				}
				DocData::MethodDoc method_doc;
				DocData::method_doc_from_methodinfo(method_doc, mi, m_func->doc_data.description);
				method_doc.is_deprecated = m_func->doc_data.is_deprecated;
				method_doc.is_experimental = m_func->doc_data.is_experimental;
				doc.methods.push_back(method_doc);
			} break;
			case GDP::ClassNode::Member::SIGNAL: {
				const GDP::SignalNode *m_signal = member.signal;
				const StringName &signal_name = m_signal->identifier->name;
				p_script->member_lines[signal_name] = m_signal->start_line;
				MethodInfo mi;
				mi.name = signal_name;
				for (const GDScriptParser::ParameterNode *p : m_signal->parameters) {
					PropertyInfo pi = _property_info_from_datatype(p->get_datatype());
					pi.name = p->identifier->name;
					mi.arguments.push_back(pi);
				}
				DocData::MethodDoc signal_doc;
				DocData::signal_doc_from_methodinfo(signal_doc, mi, m_signal->doc_data.description);
				signal_doc.is_deprecated = m_signal->doc_data.is_deprecated;
				signal_doc.is_experimental = m_signal->doc_data.is_experimental;
				doc.signals.push_back(signal_doc);