Specify focus events work on all elements in the React DOM

Generally, focus/blur events only apply to focusable elements found in forms. In React, you can listen to focus/blur events on any element.

docs/ref-05-events.md

@@ -138,6 +138,7 @@ Properties:
 DOMEventTarget relatedTarget
 ```
 
+These focus events work on all elements in the React DOM, not just form elements.
 
 ### Form Events