Document that wal_sync_method open_* methods use O_DIRECT, if available.

Backpatch to 8.2.X.

doc/src/sgml/config.sgml

-<!-- $PostgreSQL: pgsql/doc/src/sgml/config.sgml,v 1.108 2007/02/01 00:28:16 momjian Exp $ -->
+<!-- $PostgreSQL: pgsql/doc/src/sgml/config.sgml,v 1.109 2007/02/08 03:56:42 momjian Exp $ -->
 
 <chapter Id="runtime-config">
   <title>Server Configuration</title>
@@ -1385,6 +1385,7 @@ SET ENABLE_SEQSCAN TO OFF;
         Not all of these choices are available on all platforms.
         The default is the first method in the above list that is supported
         by the platform.
+        The <literal>open_</>* options also use <literal>O_DIRECT</> if available.
         This parameter can only be set in the <filename>postgresql.conf</>
         file or on the server command line.
        </para>