Updates documentaton to indicate that :expires_in can be used with all cache stores.

actionpack/lib/action_controller/caching/actions.rb

@@ -47,7 +47,8 @@ module Caching
     # And you can also use <tt>:if</tt> (or <tt>:unless</tt>) to pass a
     # proc that specifies when the action should be cached.
     #
-    # Finally, if you are using memcached, you can also pass <tt>:expires_in</tt>.
+    # As of Rails 3.0, you can also pass <tt>:expires_in</tt> with a time 
+    # interval (in seconds) to schedule expiration of the cached item.
     #
     # The following example depicts some of the points made above:
     #