Merge pull request #4779 from Polymer/fix-dom-module-docs

Fix dom-module API docs with static `import` function

lib/elements/dom-module.html

@@ -42,7 +42,7 @@
    *
    * Then in code in some other location that cannot access the dom-module above
    *
-   *     let img = document.createElement('dom-module').import('foo', 'img');
+   *     let img = customElements.get('dom-module').import('foo', 'img');
    *
    * @customElement
    * @extends HTMLElement