pgsql: Document the DISTINCT noise word in the UNION/INTERSECT/EXCEPT c

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.

Branch
------
master

Details
-------
http://git.postgresql.org/gitweb?p=postgresql.git;a=commitdiff;h=71d24466fb84bf51c479dcc85b52a0c2b71b9c50

Modified Files
--------------
doc/src/sgml/ref/select.sgml      |  126 ++++++++++++++++++++-----------------
doc/src/sgml/ref/select_into.sgml |    2 +-
2 files changed, 69 insertions(+), 59 deletions(-)