Update index.mdx (#14037)

Clarifying the deprecation message to avoid any confusion. A customer
was confused about the alert message explaining that Splunk Cloud is
supported.

Author: InterstellarStella

docs/organization/integrations/data-visualization/splunk/index.mdx

@@ -8,7 +8,7 @@ og_image: /og-images/organization-integrations-data-visualization-splunk.png
 ---
 <Alert>
 
-Splunk integration is deprecated and is not available to be installed on new projects.
+Splunk integration is deprecated and is not available to be installed on new projects. The following docs are relevant only to projects that already have the integration installed.
 
 </Alert>