Elastis update doc to rake task

doc/integration/elasticsearch.md

@@ -134,6 +134,14 @@ ID_FROM=1001 ID_TO=2000 sudo gitlab-rake gitlab:elastic:index_repositories
 ID_FROM=2001 sudo gitlab-rake gitlab:elastic:index_repositories
 ```
 
+If you want to update outdated indexes you can add parameter `UPDATE_INDEX`:
+
+```
+UPDATE_INDEX=true ID_TO=1000 sudo gitlab-rake gitlab:elastic:index_repositories
+```
+
+Keep in mind that it will scan all repositories to make sure that last commit is already indexed.
+
 To index all wikis:
 
 ```