Mention pg_dump version portability

I have applied this patch that mentions pg_dump portability to 8.1.X and
CVS HEAD.

--
  Bruce Momjian   http://candle.pha.pa.us
  EnterpriseDB    http://www.enterprisedb.com

  + If your life is a hard drive, Christ can be your backup. +
Index: pg_dump.sgml
===================================================================
RCS file: /cvsroot/pgsql/doc/src/sgml/ref/pg_dump.sgml,v
retrieving revision 1.84
diff -c -r1.84 pg_dump.sgml
*** pg_dump.sgml    6 May 2006 23:25:37 -0000    1.84
--- pg_dump.sgml    13 May 2006 16:19:04 -0000
***************
*** 645,650 ****
--- 645,661 ----
     ensure good performance.
    </para>

+   <para>
+    Because <application>pg_dump</application> is used to tranfer data
+    to newer versions of <productname>PostgreSQL</>, the output of
+    <application>pg_dump</application> can be loaded into
+    newer versions of <productname>PostgreSQL</>.  It also can read older
+    <productname>PostgreSQL</> databases.  However, it usually cannot
+    read newer <productname>PostgreSQL</> databases or produce dump output
+    that can be loaded into older database versions.  To do this, manual
+    editing of the dump file might be required.
+   </para>
+
   </refsect1>

   <refsect1 id="pg-dump-examples">