docs($rootScope): add note about watching File objects

- Add note recommending against watching `File` objects with deep watchers

Closes angular#15440

src/ng/rootScope.js

@@ -297,6 +297,8 @@ function $RootScopeProvider() {
        *   according to the {@link angular.equals} function. To save the value of the object for
        *   later comparison, the {@link angular.copy} function is used. This therefore means that
        *   watching complex objects will have adverse memory and performance implications.
+       * - This should not be used to watch for changes in objects that are
+       *   or contain [File](https://developer.mozilla.org/docs/Web/API/File) objects due to limitations with {@link angular.copy `angular.copy`}.
        * - The watch `listener` may change the model, which may trigger other `listener`s to fire.
        *   This is achieved by rerunning the watchers until no changes are detected. The rerun
        *   iteration limit is 10 to prevent an infinite loop deadlock.