[ci skip] Fix initializer tamplate

lib/generators/doorkeeper/templates/initializer.rb

@@ -62,13 +62,18 @@
   # By default this option is disabled.
   #
   # Make sure you properly setup you database and have all the required columns (run
-  # `bundle exec rails generate doorkeeper:enable_polymorphic_resource_owner`).
+  # `bundle exec rails generate doorkeeper:enable_polymorphic_resource_owner` and execute Rails
+  # migrations).
   #
   # If this option enabled, Doorkeeper will store not only Resource Owner primary key
   # value, but also it's type (class name). See "Polymorphic Associations" section of
   # Rails guides: https://guides.rubyonrails.org/association_basics.html#polymorphic-associations
   #
-  # polymorphic_resource_owner
+  # [NOTE] If you apply this option on already existing project don't forget to manually
+  # update `resource_owner_type` column in the database and fix migration template as it will
+  # set NOT NULL constraint for Access Grants table.
+  #
+  # use_polymorphic_resource_owner
 
   # If you are planning to use Doorkeeper in Rails 5 API-only application, then you might
   # want to use API mode that will skip all the views management and change the way how