pgsql: Add some minimal documentation that the SQL standard requires

Log Message:
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Add some minimal documentation that the SQL standard requires parentheses
after ONLY.

Modified Files:
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pgsql/doc/src/sgml/ref:
select.sgml (r1.116 -> r1.117)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/ref/select.sgml?r1=1.116&r2=1.117)