Added 'key_prefix' keyword argument to cache_page()

This was available before r11586, but undocumented.  It has now been
re-added with documentation and explicit support, as it seems like a useful
feature and people were using it before.



git-svn-id: http://code.djangoproject.com/svn/django/trunk@11595 bcc190cf-cafb-0310-a4f2-bffc1f526a37