Explicitely mention that some GUC variables cannot be changed at

runtime.

doc/src/sgml/runtime.sgml

 <!--
-$PostgreSQL: pgsql/doc/src/sgml/runtime.sgml,v 1.244 2004/03/05 23:16:17 tgl Exp $
+$PostgreSQL: pgsql/doc/src/sgml/runtime.sgml,v 1.245 2004/03/07 01:02:55 neilc Exp $
 -->
 
 <Chapter Id="runtime">
@@ -541,8 +541,11 @@ env PGOPTIONS='-c geqo=off' psql
 SET ENABLE_SEQSCAN TO OFF;
 </screen>
     If <command>SET</> is allowed, it overrides all other sources of
-    values for the parameter.  Superusers are allowed to <command>SET</>
-    more values than ordinary users.
+    values for the parameter.  Superusers are allowed to
+    <command>SET</> more values than ordinary users.  Some parameters
+    cannot be <command>SET</command> even by superusers: for example,
+    if they control behavior that cannot reasonably be changed without
+    restarting <productname>PostgreSQL</productname>.
    </para>
 
    <para>