Add note that the logging collector can block backends in high load situations.

doc/src/sgml/config.sgml

-<!-- $PostgreSQL: pgsql/doc/src/sgml/config.sgml,v 1.225 2009/09/08 17:08:36 tgl Exp $ -->
+<!-- $PostgreSQL: pgsql/doc/src/sgml/config.sgml,v 1.226 2009/09/10 15:02:46 alvherre Exp $ -->
 
 <chapter Id="runtime-config">
   <title>Server Configuration</title>
@@ -2422,6 +2422,19 @@ local0.*    /var/log/postgresql
          is dynamic-linker failure messages).
          This parameter can only be set at server start.
        </para>
+
+       <note>
+        <para>
+          The logging collector is designed to never lose messages.  This means
+          that in case of extremely high load, server processes could be
+          blocked due to trying to send additional log messages when the
+          collector has fallen behind.  In contrast, <application>syslog</>
+          prefers to drop messages if it cannot write them, which means it's
+          less reliable in those cases but it will not block the rest of the
+          system.
+        </para>
+       </note>
+
       </listitem>
      </varlistentry>