pgsql: Doc: improve description of UNION/INTERSECT/EXCEPT syntax.

Doc: improve description of UNION/INTERSECT/EXCEPT syntax.

queries.sgml failed to mention the rather important point that
INTERSECT binds more tightly than UNION or EXCEPT.  I thought
it could also use more discussion of the role of parentheses
in these constructs.

Per gripe from Christopher Painter-Wakefield.

Discussion: https://postgr.es/m/163338891727.12510.3939775743980651160@wrigleys.postgresql.org

Branch
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REL9_6_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/086cda1d98238abf4f94c3edb6f4004abc9de705

Modified Files
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doc/src/sgml/queries.sgml | 53 ++++++++++++++++++++++++++++++++++++++---------
1 file changed, 43 insertions(+), 10 deletions(-)