Merge branch 'master' of https://github.com/reactjs/reactjs.org into sync-a0911655

Author: reactjs-translation-bot

content/docs/typechecking-with-proptypes.md

@@ -57,6 +57,9 @@ MyComponent.propTypes = {
   // A React element.
   optionalElement: PropTypes.element,
 
+  // A React element type (ie. MyComponent).
+  optionalElementType: PropTypes.elementType,
+  
   // You can also declare that a prop is an instance of a class. This uses
   // JS's instanceof operator.
   optionalMessage: PropTypes.instanceOf(Message),