Documenting removal of nonnullvalue() and friends

Hi,

It seems that this commit:

commit 2cf8afe5d11377faaf5721b2c2f0089436abf704
Author: Peter Eisentraut <peter_e@gmx.net>
Date:   Sun Oct 5 17:33:17 2008 +0000

    Remove obsolete internal functions istrue, isfalse, isnottrue, isnotfalse,
    nullvalue, nonvalue.  A long time ago, these were used to implement the SQL
    constructs IS TRUE, etc.

removed several undocumented functions which probably weren't intended
for wide use. The commit message claims these functions are
"internal", though they were accessible as SQL-language functions.  At
least one person had been using them on 8.3, and was surprised at
seeing them gone with no explanation:
  http://archives.postgresql.org/pgsql-novice/2010-10/msg00084.php

Perhaps a mention of this change could be made in the 8.4 release notes?

Josh