Doc update for pg_start_backup

Added a note to the docs that pg_start_backup can take a long time to
finish now that we spread out checkpoints:

*** doc/src/sgml/backup.sgml    1 Feb 2007 00:28:16 -0000       2.97
--- doc/src/sgml/backup.sgml    28 Jun 2007 11:44:20 -0000
***************
*** 672,678 ****
       <para>
        It does not matter which database within the cluster you connect
to to
        issue this command.  You can ignore the result returned by the
function;
!      but if it reports an error, deal with that before proceeding.
       </para>
      </listitem>
      <listitem>
--- 672,682 ----
       <para>
        It does not matter which database within the cluster you connect
to to
        issue this command.  You can ignore the result returned by the
function;
!      but if it reports an error, deal with that before proceeding.
Note that
!      pg_start_backup can take a long time to finish. It performs a
checkpoint,
!      and if one is already running it has to wait for it to finish
first. You
!      can adjust <varname>checkpoint_completion_target</varname> to
perform the
!      checkpoints more aggressively.
       </para>
      </listitem>
      <listitem>


--
   Heikki Linnakangas
   EnterpriseDB   http://www.enterprisedb.com