Exclude #saving? from API docs

We don't have a concrete use case for it outside of AR itself.

activerecord/lib/active_record/autosave_association.rb

@@ -284,7 +284,7 @@ def changed_for_autosave?
 
     # Returns whether or not this record is already being saved outside of the
     # current autosave callback
-    def saving?
+    def saving? #:nodoc:
       @saving
     end