Fixed #2465 -- Documented the fact that select_related doesn't follow null=True

git-svn-id: http://code.djangoproject.com/svn/django/trunk@3511 bcc190cf-cafb-0310-a4f2-bffc1f526a37

docs/db-api.txt

@@ -578,6 +578,9 @@ related ``Person`` *and* the related ``City``::
     p = b.author         # Hits the database.
     c = p.hometown       # Hits the database.
 
+Note that ``select_related()`` does not follow foreign keys that have
+``null=True``.
+
 ``extra(select=None, where=None, params=None, tables=None)``
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~