Doc: sweepers only work on Active Record Models

In response to this rails issue: https://github.com/rails/rails/issues/3729

actionpack/lib/action_controller/caching/sweeping.rb

 module ActionController #:nodoc:
   module Caching
-    # Sweepers are the terminators of the caching world and responsible for expiring caches when model objects change.
+    # Sweepers are the terminators of the caching world and responsible for expiring caches when Active Record objects change.
     # They do this by being half-observers, half-filters and implementing callbacks for both roles. A Sweeper example:
     #
     #   class ListSweeper < ActionController::Caching::Sweeper

guides/source/caching_with_rails.textile

@@ -173,7 +173,9 @@ expire_fragment('all_available_products')
 
 h4. Sweepers
 
-Cache sweeping is a mechanism which allows you to get around having a ton of +expire_{page,action,fragment}+ calls in your code. It does this by moving all the work required to expire cached content into an +ActionController::Caching::Sweeper+ subclass. This class is an observer and looks for changes to an object via callbacks, and when a change occurs it expires the caches associated with that object in an around or after filter.
+Cache sweeping is a mechanism which allows you to get around having a ton of +expire_{page,action,fragment}+ calls in your code. It does this by moving all the work required to expire cached content into an +ActionController::Caching::Sweeper+ subclass. This class is an observer and looks for changes to an Active Record object via callbacks, and when a change occurs it expires the caches associated with that object in an around or after filter.
+
+TIP: Sweepers rely on the use of Active Record and Active Record Observers. The object you are observing must be an Active Record model.
 
 Continuing with our Product controller example, we could rewrite it with a sweeper like this: