adds reference to the new private repo

.github/ISSUE_TEMPLATE/docs-request-internal.yaml

@@ -1,5 +1,5 @@
 name: "Internal documentation request (Elastic employees) "
-description: Request a documentation change or enhancement.
+description: Request a documentation change or enhancement. For issues you don't want exposed to the public, use https://github.com/elastic/security-docs-internal[our private repo].
 title: "[Request] "
 body:
   - type: markdown