pgsql: Document that to_char('J'/Julian) is midnight-based, per report

Log Message:
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Document that to_char('J'/Julian) is midnight-based, per report that
Julian technically is noon to noon.

Modified Files:
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pgsql/doc/src/sgml:
func.sgml (r1.364 -> r1.365)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/func.sgml.diff?r1=1.364&r2=1.365)