Add "validate_property" virtual func

- Fix docs – `Object::validate_property` can't be called directly.

Author: Yarwin

godot-codegen/src/generator/virtual_traits.rs

@@ -119,7 +119,7 @@ fn make_special_virtual_methods(notification_enum_name: &Ident) -> TokenStream {
             unimplemented!()
         }
 
-        /// Called whenever Godot [`validate_property()`](crate::classes::Object::validate_property) is called. Allows to customize existing properties.
+        /// Called whenever Godot retrieves value of property. Allows to customize existing properties.
         /// Every property info goes through this method, except properties **added** with `get_property_list()`.
         ///
         /// Exposed `property` here is a shared mutable reference obtained (and returned to) from Godot.