Implement this method to show asset information on top of the asset preview.
You will also have to implement HasPreviewGUI and OnPreviewGUI in addition to this method.
The default implementation just returns an empty string, which disables the feature.
Note: Inspector previews are limited to the primary editor of persistent objects (assets). For example, GameObjectInspector, MaterialEditor, TextureInspector, and so on. This means that it is currently not possible for a component to have its own inspector preview.