Add mention that preload_libraries check the magic block, so

non-PostgreSQL libraries cannot be loaded using this capability.

doc/src/sgml/config.sgml

-<!-- $PostgreSQL: pgsql/doc/src/sgml/config.sgml,v 1.61 2006/06/05 03:03:42 tgl Exp $ -->
+<!-- $PostgreSQL: pgsql/doc/src/sgml/config.sgml,v 1.62 2006/06/15 17:49:09 momjian Exp $ -->
 
 <chapter Id="runtime-config">
   <title>Server Configuration</title>
@@ -961,7 +961,7 @@ SET ENABLE_SEQSCAN TO OFF;
 
        <para>
         <productname>PostgreSQL</productname> procedural language
-        libraries may be preloaded in this way, typically by using the
+        libraries can be preloaded in this way, typically by using the
         syntax <literal>'$libdir/plXXX:plXXX_init'</literal> where
         <literal>XXX</literal> is <literal>pgsql</>, <literal>perl</>,
         <literal>tcl</>, or <literal>python</>.
@@ -973,7 +973,10 @@ SET ENABLE_SEQSCAN TO OFF;
         library is first used.  However, the time to start each new
         server process may increase slightly, even if that process never
         uses the library.  So this parameter is recommended only for
-        libraries that will be used in most sessions.
+        libraries that will be used in most sessions.  The <quote>magic
+        block</> present in all PostgreSQL-supported libraries is checked
+        to guarantee compatibility.  For this reason, non-PostgreSQL
+        libraries cannot be loaded in this way.
        </para>
       </listitem>
      </varlistentry>