says "The preferred way to add a unique constraint to a table is ALTER TABLE ... ADD CONSTRAINT. The use of indexes to enforce unique constraints could be considered an implementation detail that should not be accessed directly... ".
That note has been there forever, in particular since before we supported CONCURRENTLY, which is pretty darn important in many uses nowadays. Since it seems like some users have taken this suggestion seriously, e.g. on stackoverflow[1], how about we remove this outdated suggestion? There is already a previous mention a few sections earlier[2] suggesting the use of CONCURRENTLY, so I don't think we really need to reiterate that suggestion here.