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
------
REL9_5_STABLE

Details
-------
http://git.postgresql.org/pg/commitdiff/65f1510a82a2e792b8b0c4ffacdb4e996410d943

Modified Files
--------------
doc/src/sgml/ddl.sgml | 56 ++++++++++++++++++++------------------
doc/src/sgml/ref/create_table.sgml | 26 +++++++++---------
2 files changed, 42 insertions(+), 40 deletions(-)