Document change to Range interface

docs/source/howto/how_to_create_a_custom_range.rst

@@ -27,6 +27,8 @@ A custom range must:
   boolean
 * have a ``num_products`` method that returns the number of products in the
   range or ``None`` if such a query would be too expensive.
+* have an ``all_products`` method that returns a queryset of all products in the
+  range.
 
 Example::
 
@@ -37,7 +39,10 @@ Example::
             return "!" in product.title
 
         def num_products(self):
-            return Product.objects.filter(title__icontains="!").count()
+            return self.all_products().count()
+
+        def all_products(self):
+            return Product.objects.filter(title__icontains="!")
 
 Create range instance
 ---------------------