Explain arbitrariness of ref name in callback (#8913)

* Explain arbitrariness of ref name in callback 

The sample code was confusing because it's not clear that "textInput" in this.textInput is an arbitrary name for the ref.

* Tweak wording

docs/docs/refs-and-the-dom.md

@@ -33,7 +33,7 @@ class CustomTextInput extends React.Component {
 
   render() {
     // Use the `ref` callback to store a reference to the text input DOM
-    // element in this.textInput.
+    // element in an instance field (for example, this.textInput).
     return (
       <div>
         <input