[chore] Update description for onRequestClose.

Reference to issue #489

Author: shivashp

docs/README.md

@@ -30,7 +30,8 @@ import ReactModal from 'react-modal';
   */
   onAfterOpen={handleAfterOpenFunc}
   /*
-    Function that will be run when the modal is requested to be closed, prior to actually closing.
+    Function that will be run when the modal is requested to be closed (either by clicking on overlay or pressing ESC)
+    Note: It is not called if isOpen is changed by other means.
   */
   onRequestClose={handleRequestCloseFunc}
   /*