Added a note about needing document globally available

Added a note in ``10.4-test-utils.md` about needing window, document and
document.createElement globally available before importing React.

docs/10.4-test-utils.md

@@ -49,6 +49,11 @@ ReactComponent renderIntoDocument(
 
 Render a component into a detached DOM node in the document. **This function requires a DOM.**
 
+> Note:
+>
+> You will need to have `window`, `window.document` and `window.document.createElement`
+ globally available **before** you import React. Otherwise React will think it can't access the DOM and methods like `setState` won't work.  
+
 ### mockComponent
 
 ```javascript