Update warning on xpack.fleet.enableExperimental (elastic#188917)

## Summary

Add warning on `xpack.fleet.enableExperimental` settings.

### For maintainers

- [x] Review the wording.
- [ ] Backport to all versions

(cherry picked from commit 647b222)

docs/settings/fleet-settings.asciidoc

@@ -311,4 +311,10 @@ List of proxies to access {fleet-server} that are configured when the {fleet} ap
 `xpack.fleet.enableExperimental`::
 List of experimental feature flag to enable in Fleet.
 
+[NOTE]
+====
+Experimental features should not be enabled in production environments.
+The features in this section are experimental and may be changed or removed completely in future releases.
+Elastic will make a best effort to fix any issues, but experimental features are not supported to the same level as generally available (GA) features.
+====