pgsql: Doc: clarify that CREATE TABLE discards redundant unique constra

Doc: clarify that CREATE TABLE discards redundant unique constraints.

The SQL standard says that redundant unique constraints are disallowed,
but we long ago decided that throwing an error would be too
user-unfriendly, so we just drop redundant ones. The docs weren't very
clear about that though, as this behavior was only explained for PRIMARY
KEY vs UNIQUE, not UNIQUE vs UNIQUE.

While here, I couldn't resist doing some copy-editing and markup-fixing
on the adjacent text about INCLUDE options.

Per bug #16767 from Matthias vd Meent.

Discussion: https://postgr.es/m/16767-1714a2056ca516d0@postgresql.org

Branch
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REL_10_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/567d6d1ff6173cae02a4b89b4283137bf56caf97

Modified Files
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doc/src/sgml/ref/create_table.sgml | 22 +++++++++++++++-------
1 file changed, 15 insertions(+), 7 deletions(-)