Documented +ActiveSupport::Notifications+ module.

railties/guides/source/initialization.textile

@@ -592,7 +592,11 @@ This file defines the behavior of the +ActiveSupport::Deprecation+ module, setti
 
 h4. +activesupport/lib/active_support/notifications.rb+
 
-TODO: document +ActiveSupport::Notifications+.
+This file defines the +ActiveSupport::Notifications+ module. Notifications provides an instrumentation API for Ruby, shipping with a queue implementation that consumes and publish events to log subscribers in a thread.
+
+The "API documentation":http://api.rubyonrails.org/classes/ActiveSupport/Notifications.html for +ActiveSupport::Notifications+ explains the usage of this module, including the methods that it defines.
+
+The file required in +active_support/notifications.rb+ is +active_support/core_ext/module/delegation+ which is documented in the "Active Support Core Extensions Guide":http://guides.rubyonrails.org/active_support_core_extensions.html#method-delegation.
 
 h4. +activesupport/core_ext/array/wrap+