Clarify use of #map instead of #pluck

References:
https://gitlab.com/gitlab-org/gitlab/-/commit/a93b591bd3795acfe2d27d7e957d2b0a280561de#note_333641654
https://gitlab.com/gitlab-org/gitlab/-/merge_requests/27396#note_307122761

app/models/service.rb

@@ -185,6 +185,8 @@ class Service < ApplicationRecord
   private_class_method :build_nonexistent_services_for
 
   def self.list_nonexistent_services_for(scope)
+    # Using #map instead of #pluck to save one query count. This is because
+    # ActiveRecord loaded the object here, so we don't need to query again later.
     available_services_types - scope.map(&:type)
   end
   private_class_method :list_nonexistent_services_for