Clarify globalize slug documentation further

It should now be clear that both the model's table and the model's
translation table must have a slug field, and the model must declare the
slug field translatable.

Guide.rdoc

@@ -480,8 +480,9 @@ languages. If your application only needs to be localized to one or two
 languages, you may wish to consider the {FriendlyId::SimpleI18n SimpleI18n}
 module.
 
-In order to use this module, your model's translation table must have a slug
-column and must set the field +slug+ as translatable with Globalize:
+In order to use this module, your model's table and translation table must both
+have a slug column, and your model must set the +slug+ field as translatable
+with Globalize:
 
     class Post < ActiveRecord::Base
       translates :title, :slug

lib/friendly_id/globalize.rb

@@ -13,8 +13,9 @@ module is most suitable for applications that need to be localized to many
 languages, you may wish to consider the {FriendlyId::SimpleI18n SimpleI18n}
 module.
 
-In order to use this module, your model's translation table must have a slug
-column, and you must set the field +slug+ as translatable with Globalize:
+In order to use this module, your model's table and translation table must both
+have a slug column, and your model must set the +slug+ field as translatable
+with Globalize:
 
     class Post < ActiveRecord::Base
       translates :title, :slug