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

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