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