Updated Changelog to reflect removal of :dependent => :restrict

Based on rails#2502, the option has
been removed.

guides/source/upgrading_ruby_on_rails.md

@@ -614,6 +614,10 @@ config.active_record.mass_assignment_sanitizer = :strict
 
 Rails 3.2 deprecates `vendor/plugins` and Rails 4.0 will remove them completely. While it's not strictly necessary as part of a Rails 3.2 upgrade, you can start replacing any plugins by extracting them to gems and adding them to your Gemfile. If you choose not to make them gems, you can move them into, say, `lib/my_plugin/*` and add an appropriate initializer in `config/initializers/my_plugin.rb`.
 
+### Active Record
+
+Option `:dependent => :restrict` has been removed from `belongs_to`. If you want to prevent deleting the object if there are any associated objects, you can set `:dependent => :destroy` and return `false` after checking for existence of association from any of the associated object's destroy callbacks.
+
 Upgrading from Rails 3.0 to Rails 3.1
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