Doc update, CLUSTER is not MVCC-safe

As Aidan recently pointed out on performance-list
(http://archives.postgresql.org/pgsql-performance/2007-03/msg00110.php),
our documentation doesn't mention that CLUSTER isn't MVCC-safe.

The right way to fix it is to fix CLUSTER, of course, but we should
still document the current behavior in older branches.

--
   Heikki Linnakangas
   EnterpriseDB   http://www.enterprisedb.com
Index: doc/src/sgml/ref/cluster.sgml
===================================================================
RCS file: /home/hlinnaka/pgcvsrepository/pgsql/doc/src/sgml/ref/cluster.sgml,v
retrieving revision 1.40
diff -c -r1.40 cluster.sgml
*** doc/src/sgml/ref/cluster.sgml    1 Feb 2007 00:28:18 -0000    1.40
--- doc/src/sgml/ref/cluster.sgml    9 Mar 2007 11:52:27 -0000
***************
*** 100,105 ****
--- 100,116 ----
    <title>Notes</title>

     <para>
+     <command>CLUSTER</command> loses all visibility information of tuples,
+     which makes the table look empty to any snapshot that was taken
+     before the <command>CLUSTER</command> command finished. That makes
+     <command>CLUSTER</command> unsuitable for applications where
+     transactions that access the table being clustered are run concurrently
+     with <command>CLUSTER</command>. This is most visible with serializable
+     transactions, because they take only one snapshot at the beginning of the
+     transaction, but read-committed transactions are also affected.
+    </para>
+
+    <para>
      In cases where you are accessing single rows randomly
      within a table, the actual order of the data in the
      table is unimportant. However, if you tend to access some