Add @return description

Author: aomarks

lib/legacy/legacy-element-mixin.js

@@ -517,7 +517,8 @@ export const LegacyElementMixin = dedupingMixin((base) => {
      * is contained. This is a shorthand for
      * `this.getRootNode().host`.
      * @this {Element}
-     * @return {?Node}
+     * @return {?Node} The element whose local dom within which this element is
+     * contained.
      */
     get domHost() {
       let root = wrap(this).getRootNode();