Update function component notes

packages/react-next/RELEASE_NOTES.md

@@ -57,12 +57,22 @@ TODO: document any API or functionality changes
 - Removed unused defaultProps from TeachingBubbleContent.
 - Removed rootElementRef from public api.
 
+### Function component conversions
+
+- The `ref` prop for such components no longer refers to a component class instance; instead, the ref is forwarded to the underlying DOM. 
+  - We will ensure all function components correctly return a reference to the root DOM element.
+  - For components with an imperative API, you can still access that via `componentRef`.
+  - See React's docs for [`useRef`](https://reactjs.org/docs/hooks-reference.html#useref) and [`forwardRef`](https://reactjs.org/docs/react-api.html#reactforwardref) for more on using refs with function components. 
+- The [deprecated `ReactDOM.findDOMNode` API](https://reactjs.org/docs/react-dom.html#finddomnode) can't be used to find root elements (this is a React limitation).  Instead, use `ref` as described above.
+- Class extension of most components is no longer supported. 
+  - Due to time constraints, not all components will be converted by the time of v8 release. However, they may be converted at any time in the future within a minor version.
+  - Exception: Class extension of Pickers will continue to be supported for now since the current architecture relies on it. (This will change in a future major release, but not within v8.)
+- Accessing `state` of converted components is no longer possible.
+  - If you need a former state property which is not included in the relevant `IComponentName` interface, please file an issue and we can consider adding it.
+- In your components which use the converted components, you may need to wrap certain test operations in `act` from `react-dom/test-utils`. [More details here.](https://reactjs.org/docs/test-utils.html#act)
+
 ### Others
 
-- Function component conversion.
-  - The `ref` attribute for such components no longer refers to a component class instance; instead, the ref is forwarded to the underlying DOM.
-  - Class extension of converted components is no longer possible.
-    - Even for components which have not yet been converted to function components, class extension is no longer supported
 - `ThemeProvider` is required. (new)
 - `KeytipData`/`keytipProps` removed from `Link`/`Toggle`/`Checkbox`.
 - `Button` and `Card` are new components that break from their previous implementation.