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: /messages/by-id/163338891727.12510.3939775743980651160@wrigleys.postgresql.org

Branch
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REL_12_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/b6cf89b025f82a9ab5c2961f216ca6a2751ad793

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