Mention that to_char(interval) will be removed in 8.1.

doc/src/sgml/release.sgml

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 <appendix id="release">
@@ -321,6 +321,13 @@ $PostgreSQL: pgsql/doc/src/sgml/release.sgml,v 1.286 2004/08/18 15:48:36 momjian
        ignored as well as leading whitespace (which has always been ignored).
       </para>
      </listitem>
+
+     <listitem>
+      <para>
+       The 8.1 release will remove the function <literal>to_char(interval)
+       </literal>.
+      </para>
+     </listitem>
     </itemizedlist>
    </para>
   </sect2>