Fixed #16895 -- Warned about cost of QuerySet ordering

Thanks outofculture at gmail.com for the suggestion.

docs/ref/models/options.txt

@@ -221,6 +221,12 @@ Django quotes column and table names behind the scenes.
 
         ordering = ['-pub_date', 'author']
 
+.. warning::
+
+    Ordering is not a free operation. Each field you add to the ordering
+    incurs a cost to your database. Each foreign key you add will
+    impliclty include all of its default orderings as well.
+
 ``permissions``
 ---------------
 

docs/ref/models/querysets.txt

@@ -334,6 +334,12 @@ You can tell if a query is ordered or not by checking the
 :attr:`.QuerySet.ordered` attribute, which will be ``True`` if the
 ``QuerySet`` has been ordered in any way.
 
+.. warning::
+
+    Ordering is not a free operation. Each field you add to the ordering
+    incurs a cost to your database. Each foreign key you add will
+    impliclty include all of its default orderings as well.
+
 reverse
 ~~~~~~~
 

docs/topics/db/optimization.txt

@@ -306,6 +306,17 @@ instead of::
 
    entry.blog.id
 
+Don't order results if you don't care
+-------------------------------------
+
+Ordering is not free; each field to order by is an operation the database must
+perform. If a model has a default ordering (:attr:`Meta.ordering
+<django.db.models.Options.ordering>`) and you don't need it, remove
+it on a ``QuerySet`` by calling
+:meth:`~django.db.models.query.QuerySet.order_by()` with no parameters.
+
+Adding an index to your database may help to improve ordering performance.
+
 Insert in bulk
 ==============