docs: changes Model.md to reflect known issue

Changes Model.md to reflect a known issue with property reflection when dealing with type aliases and extracted types. See loopbackio#3863. No code changes.

docs/site/Model.md

@@ -465,6 +465,10 @@ The property decorator leverages LoopBack's
 [metadata package](https://github.com/strongloop/loopback-next/tree/master/packages/metadata)
 to determine the type of a particular property.
 
+{% include note.html content="
+Currently, property types must be specified explicitly either on the property itself or via the `type` option of the property decorator. Aliased types or types that extracted from a class or interface (e.g. `public name: OtherClass['otherProperty']`) will not work properly and will result in the property type being resolved as an empty object rather than the intended type in the generated OpenAPI specifcation. This is due to a limitation and flaw in the way TypeScript currently generates the metadata that is used to generate the OpenAPI specification for the application. (see [Issue #3863](https://github.com/strongloop/loopback-next/issues/3863))
+" %}
+
 ```ts
 @model()
 class Product extends Entity {