CREATE CONSTRAINT TRIGGER

Some time ago, CREATE CONSTRAINT TRIGGER was a deprecated command and it
was slated for obsolence or removal.

Recent developments have turned it back into non-deprecated mode; it's
not going anywhere, and it needs to be fully documented.

It seems to me that it makes more sense to merge its documentation into
the CREATE TRIGGER page, where it belongs.  Right now, the documentation
page for CREATE CONSTRAINT TRIGGER is 50% a repetition of what's in the
CREATE TRIGGER page; and a significant portion of the rest is spent
telling the user to refer to the CREATE TRIGGER page.  The original
information to be found in CREATE CONSTRAINT TRIGGER is not all that
much.

Thus the attached patch.

-- 
Álvaro Herrera