Update docs for Multiple LDAP config

doc/administration/auth/ldap/index.md

@@ -413,10 +413,21 @@ gitlab_rails['ldap_servers'] = {
   'host' =>  'ad-secondary.example.net',
   'port' => 636,
   ...
+  },
+
+'tertiary' => {
+  'label' => 'GitLab Tertiary AD',
+  'host' =>  'ad-tertiary.example.net',
+  'port' => 636,
+  ...
   }
+  
 }
 ```
 
+NOTE: **Note:**
+Any number of LDAP servers can be configured. However, make sure to use a unique naming convention for the `label` section of each entry as this will be the display name of the tab shown on the sign-in page.
+
 ## User sync **(STARTER ONLY)**
 
 Once per day, GitLab runs a worker to check and update GitLab