Fixes #1998: add api doc for `customStyle` property

@@ -34,6 +34,14 @@
           [];
       },
 
+      /**
+       * An element's style properties can be directly modified by 
+       * setting key-value pairs in `customStyle` on the element 
+       * (analogous to setting `style`) and then calling `updateStyles()`.
+       *
+       */
+      customStyle: {},
+
       // here we have an instance time spot to put custom property data
       _setupStyleProperties: function() {
         this.customStyle = {};