Add an extra note to Export's diagnostic::on_unimplemented informing about #[export] of Gd<T> or DynGd<T, D>.

Author: Yarwin

godot-core/src/registry/property.rs

@@ -64,7 +64,8 @@ pub trait Var: GodotConvert {
 #[diagnostic::on_unimplemented(
     message = "`#[var]` properties require `Var` trait; #[export] ones require `Export` trait",
     label = "type cannot be used as a property",
-    note = "see also: https://godot-rust.github.io/book/register/properties.html"
+    note = "see also: https://godot-rust.github.io/book/register/properties.html",
+    note = "`Gd` and `DynGd` cannot be used directly as properties because they are non-nullable. In such a case use `OnEditor<T>` or `Option<T>` instead."
 )]
 pub trait Export: Var {
     /// The export info to use for an exported field of this type, if no other export info is specified.