Missing piece to fixing Polymer#3094

Update the documentation in the actual dom-repeat element.

Author: ktiedt

src/lib/template/dom-repeat.html

@@ -159,11 +159,12 @@
       },
 
       /**
-       * A function that can be used to filter items out of the view.  This
-       * property should either be provided as a string, indicating a method
-       * name on the element's host, or else be an actual function.  The
-       * function should match the sort function passed to `Array.filter`.
-       * Using a filter function has no effect on the underlying `items` array.
+       * `filter`. Specifies a filter callback function, that takes a single
+       * argument (the item) and returns true to display the item, false to omit
+       * it. Using a filter callback has no effect on the underlying `items`
+       * array.
+       * (Note that this is **similar** to the standard `Array`
+       * [`filter`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/filter) API, but the callback only takes a single argument.)
        */
       filter: {
         type: Function,