Merge branch 'docs/omnibus-2452-tidy-gitlab-application' into 'master'

Tidy up the documentation of Gitlab HA/Gitlab Application

See merge request gitlab-org/gitlab-ce!16011

doc/administration/high_availability/gitlab.md

 # Configuring GitLab for HA
 
-Assuming you have already configured a database, Redis, and NFS, you can
+Assuming you have already configured a [database](database.md), [Redis](redis.md), and [NFS](nfs.md), you can
 configure the GitLab application server(s) now. Complete the steps below
 for each GitLab application server in your environment.
 
@@ -56,8 +56,7 @@ for each GitLab application server in your environment.
     high_availability['mountpoint'] = '/var/opt/gitlab/git-data'
 
     # Disable components that will not be on the GitLab application server
-    postgresql['enable'] = false
-    redis['enable'] = false
+    roles ['application_role']
 
     # PostgreSQL connection details
     gitlab_rails['db_adapter'] = 'postgresql'