pgsql: Allow mixing of traditional and SQL:2008 LIMIT/OFFSET syntax.

Log Message:
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Allow mixing of traditional and SQL:2008 LIMIT/OFFSET syntax. Being rigid
about it doesn't simplify the grammar at all, and it does invite confusion
among those who only read the SELECT syntax summary and not the full details.
Per gripe from Jaime Casanova.

Modified Files:
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pgsql/doc/src/sgml/ref:
select.sgml (r1.122 -> r1.123)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/ref/select.sgml?r1=1.122&r2=1.123)
pgsql/src/backend/parser:
gram.y (r2.676 -> r2.677)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/parser/gram.y?r1=2.676&r2=2.677)