Fixed #5267 -- Documented that order_by('?') queries can be slow

git-svn-id: http://code.djangoproject.com/svn/django/trunk@6293 bcc190cf-cafb-0310-a4f2-bffc1f526a37

docs/db-api.txt

@@ -511,6 +511,9 @@ like so::
 
     Entry.objects.order_by('?')
 
+Note: ``order_by('?')`` queries may be expensive and slow, depending on the
+database backend you're using.
+
 To order by a field in a different table, add the other table's name and a dot,
 like so::