Fixed #2705: added a select_for_update() clause to querysets.

A number of people worked on this patch over the years -- Hawkeye, Colin Grady,
KBS, sakyamuni, anih, jdemoor, and Issak Kelly. Thanks to them all, and
apologies if I missed anyone.

Special thanks to Dan Fairs for picking it up again at the end and seeing this
through to commit.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@16058 bcc190cf-cafb-0310-a4f2-bffc1f526a37

docs/ref/databases.txt

@@ -359,6 +359,13 @@ store a timezone-aware ``time`` or ``datetime`` to a
 :class:`~django.db.models.TimeField` or :class:`~django.db.models.DateTimeField`
 respectively, a ``ValueError`` is raised rather than truncating data.
 
+Row locking with ``QuerySet.select_for_update()``
+-------------------------------------------------
+
+MySQL does not support the ``NOWAIT`` option to the ``SELECT ... FOR UPDATE``
+statement. If ``select_for_update()`` is used with ``nowait=True`` then a
+``DatabaseError`` will be raised.
+
 .. _sqlite-notes:
 
 SQLite notes
@@ -493,6 +500,12 @@ If you're getting this error, you can solve it by:
       This will simply make SQLite wait a bit longer before throwing "database
       is locked" errors; it won't really do anything to solve them.
 
+``QuerySet.select_for_update()`` not supported
+----------------------------------------------
+
+SQLite does not support the ``SELECT ... FOR UPDATE`` syntax. Calling it will
+have no effect.
+
 .. _oracle-notes:
 
 Oracle notes