[docs] Include ref forwarding in API design

docs/src/pages/guides/api/api.md

@@ -136,3 +136,10 @@ The Material-UI components use a combination of the two approaches according to
 - An *enum* is used when **> 2** degrees of freedom are required, or if there is the possibility that additional degrees of freedom may be required in the future.
 
 Going back to the previous button example; since it requires 3 degrees of freedom, we use an *enum*.
+
+### Ref
+
+The `ref` is forwarded to the root element. This means that without changing the rendered root element
+via `component` prop it is forwarded to the outermost DOM element that component
+renders. If you pass a different component via `component` prop the ref will be attached
+to that component instead.