docs(ngModelController): provide a more intuitive example

The example directive, using contenteditable was not showing required
even if you cleared the content from it.

Closes #3156

src/ng/directive/input.js

@@ -887,7 +887,13 @@ var VALID_CLASS = 'ng-valid',
 
               // Write data to the model
               function read() {
-                ngModel.$setViewValue(element.html());
+                var html = element.html();
+                // When we clear the content editable the browser leaves a <br> behind
+                // If strip-br attribute is provided then we strip this out
+                if( attrs.stripBr && html == '<br>' ) {
+                  html = '';
+                }
+                ngModel.$setViewValue(html);
               }
             }
           };
@@ -897,6 +903,7 @@ var VALID_CLASS = 'ng-valid',
       <form name="myForm">
        <div contenteditable
             name="myWidget" ng-model="userContent"
+            strip-br="true"
             required>Change me!</div>
         <span ng-show="myForm.myWidget.$error.required">Required!</span>
        <hr>