Fixed #10704 -- Resurrected documentation for {% else %} clause on {%...

Fixed #10704 -- Resurrected documentation for {% else %} clause on {% ifchanged %}, lost during docs refactor. Thanks to Tarken for the report.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@10563 bcc190cf-cafb-0310-a4f2-bffc1f526a37