Document the constraints method

actionpack/lib/action_dispatch/routing/mapper.rb

@@ -634,7 +634,61 @@ module ActionDispatch
                       :shallow_path => path, :shallow_prefix => path }.merge!(options)
           scope(options) { yield }
         end
-
+        
+        # === Parameter Restriction
+        # Allows you to constrain the nested routes based on a set of rules.
+        # For instance, in order to change the routes to allow for a dot character in the +id+ parameter:
+        #
+        #   constraints(:id => /\d+\.\d+) do
+        #     resources :posts
+        #   end
+        #
+        # Now routes such as +/posts/1+ will no longer be valid, but +/posts/1.1+ will be.
+        # The +id+ parameter must match the constraint passed in for this example.
+        # 
+        # You may use this to also resrict other parameters:
+        #
+        #   resources :posts do
+        #     constraints(:post_id => /\d+\.\d+) do
+        #       resources :comments
+        #     end
+        #
+        # === Restricting based on IP
+        #
+        # Routes can also be constrained to an IP or a certain range of IP addresses:
+        #
+        #   constraints(:ip => /192.168.\d+.\d+/) do
+        #     resources :posts
+        #   end
+        #
+        # Any user connecting from the 192.168.* range will be able to see this resource,
+        # where as any user connecting outside of this range will be told there is no such route.
+        #
+        # === Dynamic request matching
+        #
+        # Requests to routes can be constrained based on specific critera:
+        #
+        #    constraints(lambda { |req| req.env["HTTP_USER_AGENT"] =~ /iPhone/ }) do
+        #      resources :iphones
+        #    end
+        #
+        # You are able to move this logic out into a class if it is too complex for routes.
+        # This class must have a +matches?+ method defined on it which either returns +true+
+        # if the user should be given access to that route, or +false+ if the user should not.
+        #
+        #    class Iphone
+        #      def self.matches(request)
+        #        request.env["HTTP_USER_AGENT"] =~ /iPhone/
+        #      end
+        #    end
+        #
+        # An expected place for this code would be +lib/constraints+.
+        #
+        # This class is then used like this:
+        #
+        #    constraints(Iphone) do
+        #      resources :iphones
+        #    end
         def constraints(constraints = {})
           scope(:constraints => constraints) { yield }
         end