[1.2.X] Fixed #14464 -- Strengthened the admonition regarding documentation...

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

Backport of [14274] from trunk.

git-svn-id: http://code.djangoproject.com/svn/django/branches/releases/1.2.X@14275 bcc190cf-cafb-0310-a4f2-bffc1f526a37