Doc: remove long-obsolete advice about generated constraint names.

Doc: remove long-obsolete advice about generated constraint names.

It's been twenty years since we generated constraint names that
look like "$N".  So this advice about double-quoting such names
is well past its sell-by date, and now it merely seems confusing.

Reported-by: Yaroslav Saburov <y.saburov@gmail.com>
Author: "David G. Johnston" <david.g.johnston@gmail.com>
Discussion: https://postgr.es/m/174393459040.678.17810152410419444783@wrigleys.postgresql.org
Backpatch-through: 13