📝 update docs

Author: acontreras89

src/ScrollSync.js

@@ -9,6 +9,11 @@ import PropTypes from 'prop-types'
 export default class ScrollSync extends Component {
 
   static propTypes = {
+    /**
+     * Callback to be invoked when synchronization happens with the element that
+     * received the scroll event.
+     * Signature: `(Element)`
+     */
     onSync: PropTypes.func,
     children: PropTypes.element.isRequired,
     proportional: PropTypes.bool,