pgsql: Clarify documentation about SQL:2008 variant of LIMIT/OFFSET syn

Clarify documentation about SQL:2008 variant of LIMIT/OFFSET syntax.

The point that you need parentheses for non-constant expressions apparently
needs to be brought out a bit more clearly, per bug #6315.

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/15a5006aac1432da35ff6a7ae27a018ed300e860

Modified Files
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doc/src/sgml/ref/select.sgml |   21 ++++++++++++---------
1 files changed, 12 insertions(+), 9 deletions(-)