pgsql: interval_abs(): Add C comment about why there is no

Log Message:
-----------
interval_abs():

Add C comment about why there is no interval_abs(): it is unclear what
value to return:

http://archives.postgresql.org/pgsql-general/2009-10/msg01031.php
http://archives.postgresql.org/pgsql-general/2009-11/msg00041.php

Modified Files:
--------------
pgsql/src/backend/utils/adt:
timestamp.c (r1.203 -> r1.204)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/adt/timestamp.c?r1=1.203&r2=1.204)