Change wording so that you don't need to understand that wal_levels

form a hierarchy. Per Simon's suggestion.

doc/src/sgml/config.sgml

-<!-- $PostgreSQL: pgsql/doc/src/sgml/config.sgml,v 1.275 2010/05/03 09:14:17 heikki Exp $ -->
+<!-- $PostgreSQL: pgsql/doc/src/sgml/config.sgml,v 1.276 2010/05/03 10:31:29 heikki Exp $ -->
 
 <chapter Id="runtime-config">
   <title>Server Configuration</title>
@@ -1375,7 +1375,8 @@ SET ENABLE_SEQSCAN TO OFF;
         safely skipped, which can make those operations much faster (see
         <xref linkend="populate-pitr">). But minimal WAL does not contain
         enough information to reconstruct the data from a base backup and the
-        WAL logs, so at least <literal>archive</> level must be used to enable
+        WAL logs, so either <literal>archive</> or <literal>hot_standby</>
+        level must be used to enable
         WAL archiving (<xref linkend="guc-archive-mode">) and streaming
         replication.
        </para>