add refresh parameter to update API

Author: Paikan

guide/reference/api/update.textile

@@ -59,6 +59,7 @@ p. The update operation supports similar parameters as the index API, including:
 * @replication@: The replication type for the delete/index operation (sync or async).
 * @consistency@: The write consistency of the index/delete operation.
 * @percolate@: Enables percolation and filters out which percolator queries will be executed.
+* @refresh@: Refresh the index immediately after the operation occurs, so that the updated document appears in search results immediately.
 
 p. And also support @retry_on_conflict@ which controls how many times to retry if there is a version conflict between getting the document and indexing / deleting it. Defaults to @0@.