Add note to updateStyles regarding updates to CSS mixins

Author: TimvdLippe

lib/mixins/element-mixin.html

@@ -610,6 +610,9 @@
        *
        * These properties are retained unless a value of `null` is set.
        *
+       * Note: This function does not support updating CSS mixins.
+       * You can not dynamically change the value of an `@apply`.
+       *
        * @param {Object=} properties Bag of custom property key/values to
        *   apply to this element.
        * @return {void}