pgsql: Improve documentation about PRIMARY KEY constraints.

Improve documentation about PRIMARY KEY constraints.

Get rid of the false implication that PRIMARY KEY is exactly equivalent to
UNIQUE + NOT NULL. That was more-or-less true at one time in our
implementation, but the standard doesn't say that, and we've grown various
features (many of them required by spec) that treat a pkey differently from
less-formal constraints. Per recent discussion on pgsql-general.

I failed to resist the temptation to do some other wordsmithing in the
same area.

Branch
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REL9_1_STABLE

Details
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http://git.postgresql.org/pg/commitdiff/f6c7bfb597a365b6dd2b8e1c2e00582d39b05ba0

Modified Files
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doc/src/sgml/ddl.sgml | 58 ++++++++++++++++++++------------------
doc/src/sgml/ref/create_table.sgml | 26 ++++++++---------
2 files changed, 44 insertions(+), 40 deletions(-)