docs(autocomplete): more details on async queries (#9639)

* There was no explicit mentioning of how the way async queries are working inside of the autocomplete (only a demo)

Closes #9501

Author: devversion

src/components/autocomplete/js/autocompleteDirective.js

@@ -41,8 +41,10 @@ angular
  *   <a ng-href="api/directive/mdVirtualRepeatContainer">VirtualRepeatContainer</a> documentation.
  *
  *
- * @param {expression} md-items An expression in the format of `item in items` to iterate over
- *     matches for your search.
+ * @param {expression} md-items An expression in the format of `item in results` to iterate over
+ *     matches for your search.<br/><br/>
+ *     The `results` expression can be also a function, which returns the results synchronously
+ *     or asynchronously (per Promise)
  * @param {expression=} md-selected-item-change An expression to be run each time a new item is
  *     selected
  * @param {expression=} md-search-text-change An expression to be run each time the search text
@@ -140,6 +142,34 @@ angular
  *
  * In this example, our code utilizes `md-item-template` and `ng-messages` to specify
  *     input validation for the field.
+ *
+ * ### Asynchronous Results
+ * The autocomplete items expression also supports promises, which will resolve with the query results.
+ *
+ * <hljs lang="js">
+ *   function AppController($scope, $http) {
+ *     $scope.query = function(searchText) {
+ *       return $http
+ *         .get(BACKEND_URL + '/items/' + searchText)
+ *         .then(function(data) {
+ *           // Map the response object to the data object.
+ *           return data;
+ *         });
+ *     };
+ *   }
+ * </hljs>
+ *
+ * <hljs lang="html">
+ *   <md-autocomplete
+ *       md-selected-item="selectedItem"
+ *       md-search-text="searchText"
+ *       md-items="item in query(searchText)">
+ *     <md-item-template>
+ *       <span md-highlight-text="searchText">{{item}}</span>
+ *     </md-item-template>
+ * </md-autocomplete>
+ * </hljs>
+ *
  */
 
 function MdAutocomplete ($$mdSvgRegistry) {