Clarify that you have to install ES for source installations as well

[ci skip]

doc/integration/elasticsearch.md

@@ -35,8 +35,10 @@ These are the minimum requirements needed for Elasticsearch to work:
 
 ## Install Elasticsearch
 
-Elasticsearch is _not_ included in the Omnibus packages. Providing detailed
-information on installing Elasticsearch is out of the scope of this document.
+Elasticsearch is _not_ included in the Omnibus packages. You will have to
+install it yourself whether you are using the Omnibus package or installed
+GitLab from source. Providing detailed information on installing Elasticsearch
+is out of the scope of this document.
 
 You can follow the steps as described in the [official web site][install] or
 use the packages that are available for your OS.