Add a note about GNU tar's propensity to complain if a file changes

while tar is copying it.  This behavior is unhelpful when taking a base
backup.  Per gripe from Pallav Kalva back in April.

doc/src/sgml/backup.sgml

 <!--
-$PostgreSQL: pgsql/doc/src/sgml/backup.sgml,v 2.72 2005/10/22 21:56:07 tgl Exp $
+$PostgreSQL: pgsql/doc/src/sgml/backup.sgml,v 2.73 2005/10/22 22:09:49 tgl Exp $
 -->
 <chapter id="backup">
  <title>Backup and Restore</title>
@@ -676,6 +676,15 @@ SELECT pg_stop_backup();
   </orderedlist>
    </para>
 
+   <para>
+    Some versions of GNU <application>tar</> consider it an error if a file
+    is changed while <application>tar</> is copying it.  This case is normal,
+    and not an error, when taking a base backup of an active database.
+    Since ignoring all errors from <application>tar</> would be unwise,
+    you should either manually inspect <application>tar</>'s messages or
+    else use a different backup tool that does not complain about this case.
+   </para>
+
    <para>
     It is not necessary to be very concerned about the amount of time elapsed
     between <function>pg_start_backup</> and the start of the actual backup,