Add note that not all SQL commands support ONLY in the same way.

doc/src/sgml/ddl.sgml

-<!-- $PostgreSQL: pgsql/doc/src/sgml/ddl.sgml,v 1.84 2009/01/07 22:40:49 tgl Exp $ -->
+<!-- $PostgreSQL: pgsql/doc/src/sgml/ddl.sgml,v 1.85 2009/01/08 12:47:58 petere Exp $ -->
 
 <chapter id="ddl">
  <title>Data Definition</title>
@@ -2136,6 +2136,23 @@ VALUES ('New York', NULL, NULL, 'NY');
    permissions on it.
   </para>
 
+  <para>
+   More generally, note that not all SQL commands are able to work on
+   inheritance hierarchies.  Commands that are used for data querying,
+   data modification, or schema modification
+   (e.g., <literal>SELECT</literal>, <literal>UPDATE</literal>, <literal>DELETE</literal>,
+   most variants of <literal>ALTER TABLE</literal>, but
+   not <literal>INSERT</literal> and <literal>ALTER TABLE ...
+   RENAME</literal>) typically default to including child tables and
+   support the <literal>ONLY</literal> notation to exclude them.
+   Commands that do database maintenance and tuning
+   (e.g., <literal>REINDEX</literal>, <literal>VACUUM</literal>)
+   typically only work on individual, physical tables and do no
+   support recursing over inheritance hierarchies.  The respective
+   behavior of each individual command is documented in the reference
+   part (<xref linkend="sql-commands">).
+  </para>
+
   <para>
    A serious limitation of the inheritance feature is that indexes (including
    unique constraints) and foreign key constraints only apply to single