Doc update for back-branches, CLUSTER and MVCC-safety

Heikki Linnakangas wrote:

I posted a doc update a while ago to mention that CLUSTER isn't MVCC-safe:

http://archives.postgresql.org/pgsql-patches/2007-03/msg00137.php

CLUSTER is now MVCC-safe in CVS HEAD, but the documentation for older
releases should still be updated.

Applied to back-branches, thanks for the reminder.

//Magnus