docs: specify class name for which to implement the methods

Author: ugur-vaadin

articles/components/tree-grid/index.adoc

@@ -144,7 +144,10 @@ include::{root}/src/main/java/com/vaadin/demo/component/treegrid/TreeGridScrollT
 ----
 --
 
-`TreeDataProvider` supports this feature out of the box. For other data providers, you will need to implement [methodname]#getParent(T item)# and [methodname]#getItemIndex(T item, HierarchicalQuery query)#.
+`TreeDataProvider` supports this feature out of the box. For other data providers, you need to override and implement the following methods of [classname]`HierarchicalDataProvider`:
+
+- [methodname]#getParent(T item)#
+- [methodname]#getItemIndex(T item, HierarchicalQuery query)#
 
 === Scrolling to an Item by Path