pgsql: Clarify bit numbering in get_bit/set_bit etc.

Log Message:
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Clarify bit numbering in get_bit/set_bit etc. Per gripe from
Boszormenyi Zoltan.

Modified Files:
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pgsql/doc/src/sgml:
func.sgml (r1.527 -> r1.528)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/func.sgml?r1=1.527&r2=1.528)