Document the DISTINCT noise word in the UNION/INTERSECT/EXCEPT constructs.

Document the DISTINCT noise word in the UNION/INTERSECT/EXCEPT constructs.

I also rearranged the order of the sections to match the logical order
of processing steps: the distinct-elimination implied by SELECT DISTINCT
happens before, not after, any UNION/INTERSECT/EXCEPT combination.

Per a suggestion from Hitoshi Harada.