Clarify reason for setTextContent helper (facebook#11813)

* Update comment on setTextContent

update the comment explaining the reason for the helper

* Use `setTextContent` in ReactDOM for consistency

packages/react-dom/src/client/ReactDOM.js

@@ -43,6 +43,7 @@ import warning from 'fbjs/lib/warning';
 import * as ReactDOMComponentTree from './ReactDOMComponentTree';
 import * as ReactDOMFiberComponent from './ReactDOMFiberComponent';
 import * as ReactInputSelection from './ReactInputSelection';
+import setTextContent from './setTextContent';
 import validateDOMNesting from './validateDOMNesting';
 import * as ReactBrowserEventEmitter from '../events/ReactBrowserEventEmitter';
 import * as ReactDOMEventListener from '../events/ReactDOMEventListener';
@@ -736,7 +737,7 @@ const DOMRenderer = ReactFiberReconciler({
     },
 
     resetTextContent(domElement: Instance): void {
-      domElement.textContent = '';
+      setTextContent(domElement, '');
     },
 
     commitTextUpdate(

packages/react-dom/src/client/setTextContent.js

@@ -3,21 +3,22 @@
  *
  * This source code is licensed under the MIT license found in the
  * LICENSE file in the root directory of this source tree.
+ *
+ * @flow
  */
 
 import {TEXT_NODE} from '../shared/HTMLNodeType';
 
 /**
- * Set the textContent property of a node, ensuring that whitespace is preserved
- * even in IE8. innerText is a poor substitute for textContent and, among many
- * issues, inserts <br> instead of the literal newline chars. innerHTML behaves
- * as it should.
+ * Set the textContent property of a node. For text updates, it's faster
+ * to set the `nodeValue` of the Text node directly instead of using
+ * `.textContent` which will remove the existing node and create a new one.
  *
  * @param {DOMElement} node
  * @param {string} text
  * @internal
  */
-let setTextContent = function(node, text) {
+let setTextContent = function(node: Element, text: string): void {
   if (text) {
     let firstChild = node.firstChild;