docs(ngInit): add note on best practices

Author: btford

src/ng/directive/ngInit.js

@@ -6,23 +6,42 @@
  * @restrict AC
  *
  * @description
- * The `ngInit` directive specifies initialization tasks to be executed
- *  before the template enters execution mode during bootstrap.
+ * The `ngInit` directive allows you to evaluate an expression in the
+ * current scope.
+ *
+ * <div class="alert alert-error">
+ * The only appropriate use of `ngInit` for aliasing special properties of
+ * {@link api/ng.directive:ngRepeat `ngRepeat`}, as seen in the demo bellow. Besides this case, you
+ * should use {@link guide/dev_guide.mvc.understanding_controller controllers} rather than `ngInit`
+ * to initialize values on a scope.
+ * </div>
  *
  * @element ANY
  * @param {expression} ngInit {@link guide/expression Expression} to eval.
  *
  * @example
    <doc:example>
      <doc:source>
-    <div ng-init="greeting='Hello'; person='World'">
-      {{greeting}} {{person}}!
-    </div>
+   <script>
+     function Ctrl($scope) {
+       $scope.list = [['a', 'b'], ['c', 'd']];
+     }
+   </script>
+   <div ng-controller="Ctrl">
+     <div ng-repeat="innerList in list" ng-init="outerIndex = $index">
+       <div ng-repeat="value in innerList" ng-init="innerIndex = $index">
+          <span class="example-init">list[ {{outerIndex}} ][ {{innerIndex}} ] = {{value}};</span>
+       </div>
+     </div>
+   </div>
      </doc:source>
      <doc:scenario>
-       it('should check greeting', function() {
-         expect(binding('greeting')).toBe('Hello');
-         expect(binding('person')).toBe('World');
+       it('should alias index positions', function() {
+         expect(element('.example-init').text())
+           .toBe('list[ 0 ][ 0 ] = a;' +
+                 'list[ 0 ][ 1 ] = b;' +
+                 'list[ 1 ][ 0 ] = c;' +
+                 'list[ 1 ][ 1 ] = d;');
        });
      </doc:scenario>
    </doc:example>