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, 3b17efdf - 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.