DEV: Add comment about `UserController#cards` route (#16882)

This is unusual because it is not used directly by any core UI. It was added for theme components.

app/controllers/users_controller.rb

@@ -105,6 +105,7 @@ class UsersController < ApplicationController
     show(for_card: true)
   end
 
+  # This route is not used in core, but is used by theme components (e.g. https://meta.discourse.org/t/144479)
   def cards
     return redirect_to path('/login') if SiteSetting.hide_user_profiles_from_public && !current_user