Merge pull request #3013 from akheron/patch-2

Document React.addons.TestUtils.isElement()

docs/10.4-test-utils.md

@@ -43,6 +43,14 @@ object mockComponent(function componentClass, string? mockTagName)
 
 Pass a mocked component module to this method to augment it with useful methods that allow it to be used as a dummy React component. Instead of rendering as usual, the component will become a simple `<div>` (or other tag if `mockTagName` is provided) containing any provided children.
 
+### isElement
+
+```javascript
+boolean isElement(ReactElement element)
+```
+
+Returns true if `element` is any ReactElement.
+
 ### isElementOfType
 
 ```javascript