config.threasafe! deprecated waning in config/environments/production.rb

Since Rails 4.0 config.threadsafe! is enabled by default.
We have to strongly warn to users by not only showing deprecated warnings when
user use this option, but also writing deprecated warnings in config file.

railties/lib/rails/generators/rails/app/templates/config/environments/production.rb.tt

@@ -66,6 +66,9 @@
   # config.action_mailer.raise_delivery_errors = false
 
   # Enable threaded mode.
+  # config.threadsafe! is deprecated. Rails application behave
+  # by default as thread safe in production as long as config.cache_classes and
+  # config.eager_load are set to true.
   # config.threadsafe!
 
   # Enable locale fallbacks for I18n (makes lookups for any locale fall back to