[15.x] Update docs to use async/await (#667) (#668)

Backports the following commits to 15.x:
 - Update docs to use async/await  (#667)

docs/_examples/search.asciidoc

@@ -1,18 +1,16 @@
 .Search with a simple query string query
 [source,js]
 ---------
-client.search({
+const response = await client.search({
   index: 'myindex',
   q: 'title:test'
-}, function (error, response) {
-  // ...
 });
 ---------
 
 .Passing a full request definition in the Elasticsearch's Query DSL as a `Hash`
 [source,js]
 ---------
-client.search({
+const response = await client.search({
   index: 'myindex',
   body: {
     query: {
@@ -28,7 +26,5 @@ client.search({
       }
     }
   }
-}, function (error, response) {
-  // ...
 });
----------
+---------