Re: incorrect (incomplete) description for "alter domain"

I wrote:
> I think the page is technically correct, but I'm inclined to duplicate
> this text from the CREATE DOMAIN page:

>       where domain_constraint is:
>       [ CONSTRAINT constraint_name ]
>       { NOT NULL | NULL | CHECK (expression) }

> rather than making readers go look that up.

Actually, there *is* a bug in the description, because experimentation
shows that CREATE DOMAIN accepts NULL in this syntax (as advertised)
but ALTER DOMAIN does not.  We could alternatively decide that that's
a code bug and make ALTER DOMAIN take it, but I don't think it's worth
any effort (and this behavior may actually have been intentional, too).
I think we should just add

        where domain_constraint is:

        [ CONSTRAINT constraint_name ]
        { NOT NULL | CHECK (expression) }

to the ALTER DOMAIN page, and then remove the claim that it's
identical to CREATE DOMAIN.