docs(NgModelController): document $viewChangeListeners property

Closes angular#4948

Author: Caitlin Potter

src/ng/directive/input.js

@@ -840,6 +840,11 @@ var VALID_CLASS = 'ng-valid',
  *      }
  *      ngModel.$formatters.push(formatter);
  *      </pre>
+ *
+ * @property {Array.<Function>} $viewChangeListeners Array of functions to execute whenever the
+ *     view value has changed. It is called with no arguments, and its return value is ignored.
+ *     This can be used in place of additional $watches against the model value.
+ *
  * @property {Object} $error An object hash with all errors as keys.
  *
  * @property {boolean} $pristine True if user has not interacted with the control yet.