pgsql: Teach psql to display the comments on conversions and domains.

Teach psql to display the comments on conversions and domains.

\dc and \dD now accept a "+" option, which will cause the comments to
be displayed.  Along the way, correct a few oversights in the previous
commit in this area, 3b17efdfdd846c9bfad1637686e6f18198ea3df5 - namely,
(1) when \dL+ is used, make description still be the last column, for
consistency with what we've done elsewhere; and (2) document the
difference between \dC and \dC+.

Josh Kupershmidt, with a couple of doc changes by me.

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/c9ac00e6ecfaad4d6a3e1ee478b912c7227ae62a

Modified Files
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doc/src/sgml/ref/psql-ref.sgml |   10 +++++-
src/bin/psql/command.c         |    4 +-
src/bin/psql/describe.c        |   68 ++++++++++++++++++++++++++++-----------
src/bin/psql/describe.h        |    4 +-
src/bin/psql/help.c            |    4 +-
5 files changed, 63 insertions(+), 27 deletions(-)