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

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

Author: wesleycho

src/ng/rootScope.js

@@ -296,7 +296,7 @@ function $RootScopeProvider() {
        * - When `objectEquality == true`, inequality of the `watchExpression` is determined
        *   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.
+       *   watching complex objects will have adverse memory and performance implications. This should not be used to watch for objects that are or contain `File` objects.
        * - 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.