pgsql: Make clearer warning about using 'now' with DEFAULT: SELECT

Log Message:
-----------
Make clearer warning about using 'now' with DEFAULT:

SELECT CURRENT_TIMESTAMP;
SELECT now();
SELECT TIMESTAMP 'now'; -- incorrect for use with DEFAULT

Modified Files:
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pgsql/doc/src/sgml:
func.sgml (r1.237 -> r1.238)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/func.sgml.diff?r1=1.237&r2=1.238)