This is how it was documented originally, but several years ago somebody
decided that DEFAULT isn't a type of constraint.  Well, the grammar thinks
it is.  The documentation was wrong in two ways: it alleged that DEFAULT
had to appear before any other kind of constraint, and it alleged that you
can't prefix a DEFAULT clause with a "CONSTRAINT name" clause, when in fact
you can.  (The latter behavior probably isn't SQL-standard, but our grammar
has always allowed it.)

This patch responds to Fujii Masao's observation that the ALTER TABLE
documentation mistakenly implied that you couldn't include DEFAULT in
ALTER TABLE ADD COLUMN; though this isn't the way he proposed fixing it.