docs: further clarify getNodeKey usage

Author: fritz-c

README.md

@@ -44,7 +44,7 @@ Prop                      | Type           | <div style="width: 400px;">Descript
 :-------------------------|:--------------:|:----------------------------------------
 treeData<br/>_(required)_ | object[]       | Tree data with the following keys: <div>`title` is the primary label for the node.</div><div>`subtitle` is a secondary label for the node.</div><div>`expanded` shows children of the node if true, or hides them if false. Defaults to false.</div><div>`children` is an array of child nodes belonging to the node.</div><div>__Example__: `[{title: 'main', subtitle: 'sub'}, { title: 'value2', expanded: true, children: [{ title: 'value3') }] }]`
 onChange<br/>_(required)_ | func           | Called whenever tree data changed. Just like with React input elements, you have to update your own component's data to see the changes reflected.<div>`( treeData: object[] ): void`</div>
-getNodeKey<br/>_(recommended)_ | func      | Specify the unique key used to identify each node and generate the `path` array passed in callbacks. For example, in callbacks this will let you easily determine that the node with an `id` of `35` is (or has just become) a child of the node with an `id` of `12`, which is a child of ... and so on. It uses [`defaultGetNodeKey`](https://github.com/fritz-c/react-sortable-tree/blob/master/src/utils/default-handlers.js) by default, which returns the index in the tree (omitting hidden nodes).<div>`({ node: object, treeIndex: number }): string or number`</div>
+getNodeKey<br/>_(recommended)_ | func      | Specify the unique key used to identify each node and generate the `path` array passed in callbacks. With a setting of `getNodeKey={({ node }) => node.id}`, for example, in callbacks this will let you easily determine that the node with an `id` of `35` is (or has just become) a child of the node with an `id` of `12`, which is a child of ... and so on. It uses [`defaultGetNodeKey`](https://github.com/fritz-c/react-sortable-tree/blob/master/src/utils/default-handlers.js) by default, which returns the index in the tree (omitting hidden nodes).<div>`({ node: object, treeIndex: number }): string or number`</div>
 generateNodeProps         | func           | Generate an object with additional props to be passed to the node renderer. Use this for adding buttons via the `buttons` key, or additional `style` / `className` settings.<div>`({ node: object, path: number[] or string[], treeIndex: number, lowerSiblingCounts: number[], isSearchMatch: bool, isSearchFocus: bool }): object`</div>
 onMoveNode                | func           | Called after node move operation. <div>`({ treeData: object[], node: object, nextParentNode: object, prevPath: number[] or string[], prevTreeIndex: number, nextPath: number[] or string[], nextTreeIndex: number }): void`</div>
 onVisibilityToggle        | func           | Called after children nodes collapsed or expanded. <div>`({ treeData: object[], node: object, expanded: bool, path: number[] or string[] }): void`</div>