Fixed #14464 -- Strengthened the admonition regarding documentation versions...

Fixed #14464 -- Strengthened the admonition regarding documentation versions at the end of the install docs. Thanks to PaulM for the report.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@14274 bcc190cf-cafb-0310-a4f2-bffc1f526a37