Documentation Update: Document pg_start_backup checkpoint behavior

Hi,

small patch for the documentation describing the current pg_start_backup
checkpoint behavior as per
http://archives.postgresql.org//pgsql-general/2008-09/msg01124.php .

Should we note down a TODO to revisit the current checkpoint handling?

best regards,
Michael
diff --git a/doc/src/sgml/backup.sgml b/doc/src/sgml/backup.sgml
index 02545f1..6ea9488 100644
--- a/doc/src/sgml/backup.sgml
+++ b/doc/src/sgml/backup.sgml
@@ -737,12 +737,8 @@ SELECT pg_start_backup('label');
      (see the configuration parameter
      <xref linkend="guc-checkpoint-completion-target">).  Usually
      this is what you want because it minimizes the impact on query
-     processing.  If you just want to start the backup as soon as
-     possible, execute a <command>CHECKPOINT</> command
-     (which performs a checkpoint as quickly as possible) and then
-     immediately execute <function>pg_start_backup</>.  Then there
-     will be very little for <function>pg_start_backup</>'s checkpoint
-     to do, and it won't take long.
+     processing.  Unfortunately it's currently not possible to expedite
+     the checkpointing done by pg_start_backup.
     </para>
    </listitem>
    <listitem>