Add comment about uninstalling Cilium application

app/models/clusters/applications/cilium.rb

@@ -8,6 +8,11 @@ module Clusters
       include ::Clusters::Concerns::ApplicationCore
       include ::Clusters::Concerns::ApplicationStatus
 
+      # Cilium can only be installed and uninstalled through the
+      # cluster-applications project by triggering CI pipeline for a
+      # management project. UI operations are not available for such
+      # applications. More information:
+      # https://docs.gitlab.com/ee/user/clusters/management_project.html
       def allowed_to_uninstall?
         false
       end